A Service Guide for the Theoben Rapid MKII

Recently, I took my Theoben Rapid MKII out of its case to give it a shake down test ahead of some field work. It’s been quite a while since I have used this particular rifle. In fact I have been neglecting my rifle collection in favour of pistols for a good year or so. I know, it really is unforgivable to ignore such a superb air rifle. Nonetheless, out she came. Still full of air as the day I had carefully packed her away. I cocked the bolt back and checked that she was not loaded. Of course it wasn’t but you can never be too cautious. I pushed the bolt forwards and whilst pointing it in a safe direction, I pulled the trigger. Nothing happened. I tried again. Still nothing. Oh dear, I have a cocked rifle with no means to discharge it!

I adjusted the trigger to no avail but did manage to discharge it eventually. It became apparent that the trigger had become sticky. The lubricant must have dried out over time. I would need to disassemble, lubricate and adjust the trigger. If this lubricant has dried out, then there was a good chance that there could be other problems ahead. I decided to take the plunge and fully service the rifle. I thought it would also a good topic for another Jimmie Dee’s Airguns article.

Buddy bottle fitted with Dr Bob’s Quick Fill adaptor and gauge.

I have owned this rifle from new since the end of 2002. It was my first PCP and was all the rage back then. In fact it was a Christmas present to myself. The only modifications that I have made to this rifle is the addition of a Dr Bob’s Quick Fill Kit and I also polished the hammer and bolt. The quick fill kit is a must have addition. It allows the use of a Foster snap on coupling which eliminates the need to remove the buddy bottle when refilling. This in turn prevents burst or prematurely worn bottle o-rings and thus potential leaks. It also incorporates a mini pressure gauge. Something Theoben should have had the foresight to add to begin with! “Hindsight is a wonderful thing, but foresight is better…” – William Blake (1757 – 1827).

The beauty of this rifle is its simplicity, not to mention its precision. With just a few basic tools and some o-rings you can almost fully service it yourself. Only the regulator needs specialist tools which you can buy for a few pounds online. But there is one part of the regulator that requires a lathe, or if like me you don’t have a lathe, there are a few Theoben enthusiasts who can service this particular part for you for a reasonable fee. To be fair it isn’t a complicated job. Anyone with a lathe and some delrin rod can do this job.

A word of warning before you jump in though. Theoben fitted various designs of trigger to the Rapid. This particular rifle has the Theoben Sporter trigger and as such, I shall only describe how to service and setup this type of trigger. Just for good measure, this trigger unit also went through various evolutions of design. If your rifle has a different trigger unit, be sure to source the information needed to service it before stripping it or the rifle down. Leave a message if you need help finding the information needed as I may be able to point you in the right direction. Other than that, I believe the Rapid MKII doesn’t vary much in design.

The Theoben Rapid MKII jigsaw puzzle

safety first!

High pressure air is exceptionally dangerous when mishandled. It must be treated with respect at all times as it can be potentially explosive. This isn’t something the air rifle manufacturers tend to tell prospective buyers of PCP air guns. Always use the manufacturer’s recommended lubricants for o-rings. Any lubricant containing petrochemicals is potentially readily combustible in a high pressure air environment. At atmospheric pressure, this isn’t an issue. But as the oxygen content of air is effectively much higher when compressed to such high pressures used in an air rifle, it is especially dangerous. Having said that, providing you use the correct lubricants and filling procedures, all should be well.

Whenever you use or service an air gun, always ensure it is not loaded. Check it, then check it again, then check it again! Never be complacent with an air gun. Accidents do happen through complacency and sometimes they can be lethal.

If you have any doubts about your capabilities with respect to air rifle servicing, stop now. Take it to a competent gunsmith.

work environment and tools

It’s important to ensure that you have a good working environment to service your air rifle. A solid workbench with enough surface area to place your rifle without tools getting in the way. Place an old blanket on the work surface to protect the finish of the rifle as you are working on it. Instead of a blanket, I prefer to use a sheet of neoprene or a foam exercise mat.

Portable 6″ vice

You will also need a good vice to hold the Rapid’s block firmly whilst you use a spanner to remove the regulator. I can’t stress enough to make sure you use a good vice for this. You don’t want the block or the vice slipping as you loosen or tighten the regulator. Also, use soft faced jaws on your vice. You don’t want to end up with vice marks embedded in your pride and joy. If your vice doesn’t have soft faced jaws, you can buy slip on fibre grips that will do the job just as well. I also use a rag to further protect the finish of the block.

Slide-on fibre vice grips

Soft faced jaws alone are not sufficient to protect the finish of the block as grit and metal filings can become embedded in the material of the soft jaws. I use a 6” portable woodwork vice for small jobs. You can screw on some wood blocks to form soft jaw faces or use a pair of slide on some fibre jaws.

When working with o-rings and associated parts, make sure your work area is clean and dust free. You can use paper to put the o-rings on as a clean surface rather than directly onto the work surface.

Ziplock bags also come in very handy to keep parts for sub assemblies together.

To service the Theoben Rapid MKII you will need the following tools:

  1. I found the following allen keys were needed to service my Rapid: 3/16, 5/32, 1/8, 3/32, 5/64, 1/16 and 5mm. Theoben used imperial allen bolts on the Rapid but one in particular had a much better fit with a 5mm allen key. If you find your allen key is a sloppy fit, either it’s worn out or it’s worth trying some other sizes either metric or imperial to get a good fit. Loose fitting allen keys will round out sockets very easily. It is important to buy quality allen keys otherwise you may find that you round out the sockets. This is something you really want to avoid. Try not to use ball end style allen keys as they offer less contact surface and could cause rounded sockets easier than you think. Buy from a reputable tool shop rather than eBay or market stalls.
  2. A 21mm open spanner to remove the regulator. I advise against using adjustable spanners. When used properly they can be ok but if incorrectly adjusted they will damage the bolt head or in this case the regulator.
  3. A 10mm ring spanner to adjust the regulator.
  4. Circlip pliers.
  5. A regulator tester from Best Fittings or Dr Bob.
  6. An o-ring pick.
  7. A 1/16 brass pin punch or perhaps just a matchstick. An allen key may suffice as long as no force it necessary to remove the trigger pivot pins.
  8. Kitchen roll to clean o-rings (kitchen roll does not leave fibres whereas toilet roll does).
  9. A sheet of paper to help adjust the barrel position.
  10. A lint free cloth for general cleaning of the rifle’s exterior.
  11. Napier VP90 gun cleaner and lubricant spray or your preferred protectant – note this is supplied in the pull through kit below.
  12. Napier Airgun Pull Through Kit – if you wish to clean the rifling of the barrel.
  13. Molykote 33 o-ring lubricant as recommended by Theoben.
  14. Molybdenum grease (Bisley) as recommended by Theoben for lubricating the bolt.
  15. Abbey SM50 gun oil to lubricate the trigger pins.
  16. O-rings. Either buy an o-ring service kit from eBay or buy your own o-rings individually online. The sizes required are listed below and all should be Nitrile 70 Shore hardness unless individually stated otherwise:
    1. Valve body large o-ring: BS013 (10.82 x 1.78mm) – 1 or 2 off depending on valve body design
    2. Valve body small o-ring: 4.1 x 1.6mm – 1 off.
    3. Valve stem: BS607 (2.54 x 1.02mm) – 1 off (some valves have two o-ring grooves but only one should be fitted to the central groove).
    4. Valve cap: BS015 (14 x 1.78mm) – 1 off
    5. Regulator body: BS015 (14 x 1.78mm) – 1 off
    6. Regulator core: BS803 (6.35 x 1.78mm) – 1 off
    7. Regulator core: BS005 (2.57 x 1.78mm) – 1 off
    8. Regulator pin disc seal: BS804 (7.94 x 1.78mm) – 1 off
    9. Bolt handle: 14.6 x 2.4mm – 1 off
    10. Buddy bottle: BS111 (10.77 x 2.62mm) 90 Shore – 1 off or 2 if you have a Dr Bob’s Quick Fill adaptor. Sometimes it is necessary to fit BS112 o-rings to the Dr Bob’s quick fill adaptor.

common faults

It’s not always necessary to strip every part of the Rapid. If you are able to isolate the fault, you can strip just that part and service it. But, to be honest, o-rings are cheap and if it has been a while since it was last serviced, it’s best to replace the lot as no doubt another may fail shortly afterwards.

Here’s a list of common faults for the Rapid and how to diagnose them:

  1. Leaks: the rifle can leak air from any o-ring from the seal in the bottle all the way to the valve. A leak from the valve can be found by removing the moderator and fitting a balloon over the barrel. If it inflates, you’ve found your leak. Of course this can take a long time. A quicker way is to remove the moderator and, with the bolt locked forward, place the muzzle of the barrel into a bowl of water and look for a stream of bubbles. Just be sure to dry the barrel and rifling thoroughly afterwards! You could also just put a wet finger over the muzzle and watch for bubbles. A leak from the valve may be due to a bent valve stem or scratched or dirty valve body and valve stem faces. If a scratch is the cause, the valve body and stem will need refacing using a lathe, or worst the valve body and stem replaced entirely. It is also possible that the large valve body o-ring (sometimes two are fitted depending on the valve body design) is leaking allowing air to pass around the outside of the valve body and into the transfer port to the barrel. Other leaks can often be found by wiping a diluted soapy solution with a soft paint brush around the joints where the other o-rings are fitted. For example the bottle to regulator junction, valve cap, regulator to block junction or the blanking plug on the bottom of the block. There is also a vent hole on the regulator body. This can be checked for leaks with soapy water too. If there is a leak from this vent hole, then one or both of the regulator core o-rings have failed.
  2. Low shot count: the rifle isn’t losing air overnight but the expected number of shots per fill is very low. This can be caused by either a faulty valve stem o-ring or the smaller valve body o-ring. Often this fault is associated with a much louder report than usual on firing as air escapes past the valve stem or the rear of the valve body into the hammer cavity. A .22 12ft/lb Rapid MKII has a typical shot count of 420 whereas a .177 has a typical shot count of 220 for a 400cc bottle.
  3. Popping sound: when firing, a popping sound is heard. This is often caused by air escaping out of the breech around the bolt probe seal. Correct probe adjustment should rectify this problem. It may also be due to a leak around the valve stem.
  4. Inconsistent power: this can be caused by a faulty regulator. It can also be caused by oil or grease on the hammer. The hammer should be run dry without any lubricant. If the regulator is suspected, it should be tested using a regulator tester from either Best Fittings or Dr Bob. A faulty regulator will either be slow to return to its set pressure and/or will creep up in pressure when left on test overnight. The creep can be caused by the delrin nose seal on the regulator core, a scratch on the rear of the pin seal disc that seals on the regulator core nose or the o-ring behind the pin seal disc has hardened and/or perished. A faulty regulator can also be the cause of poor grouping.
  5. Hammer won’t cock: the cocking dog may be worn such the bolt does not retract the hammer. This would be obvious as the bolt would have no tension when retracting it. The other possibility is that the trigger has been incorrectly adjusted and the sear lever no longer has enough spring tension to hold the hammer back against the force of the hammer spring.
  6. Hammer won’t release / gun won’t fire: the trigger sear spring has been incorrectly adjusted or the sear plunger or other parts of the trigger have become sticky due to dried up lubricant.
  7. Bolt difficult to lock forward: the probe tension may have been incorrectly set. This can be checked by removing the magazine and testing to see if the problem persists. It can also just be larger sized pellets or harder lead pellets.
  8. Bolt feels “gritty” when cocking: dirt on the bolt or hammer. Strip, clean and refit.
  9. Poor grouping: this can be caused by a number of faults. It may be a fouled barrel which can be cleaned using a pull through. After using a pull through, the barrel may need to be leaded up by shooting 100 or so pellets. Poor grouping can also be caused by a change of pellet either by the shooter or the manufacturer by changing the pellet’s design slightly. It can also be caused by a faulty moderator causing the pellets to clip on exit. This can be checked by removing the moderator, tapping the end onto your hand and looking for shards of lead. Another cause of poor grouping or accuracy can be a loose barrel and/or scope. Also a faulty regulator can also affect grouping. Of course you might just be having a bad day or need to lay off the coffee!

lubricating o-rings

Before we get started on the actual strip down, it is important to know how to handle and lubricate o-rings properly. Failure to do so will result in more leaks and frustration. If you get it right first time, happy days!

Using an o-ring picker to remove o-rings

Always remove o-rings using an o-ring picker. This is similar to a dental pick but with flattened and smooth ends to reduce the risk of scoring o-ring grooves. Always clean the o-ring groove with kitchen roll as this does not leave lint whereas toilet roll does. Do not be tempted to use cotton buds as they will leave lint behind. I also use makeup sponge sticks to get into o-ring grooves where kitchen roll doesn’t work so well. You may also find it helpful to slightly dampen the kitchen roll.

Makeup sponge sticks

Similarly, clean your o-rings by pulling the o-ring rubber through a folded piece of kitchen roll. I hold the o-ring in one hand, the towel in the other, folded between finger and thumb around part of the o-ring. Then rotate and pull the o-ring through the towel.

I also use a clean sheet of kitchen towel to put the o-rings on once they are clean.

Theoben recommended that all o-rings fitted to the Rapid should be lubricated with Molykote 33 grease.

Molykote 33 – Theoben recommended o-ring grease

I would not advise using any other grease, especially petroleum or hydrocarbon based greases as these can cause explosions due to the high oxygen concentration in compressed air. You should use the grease sparingly and apply only a thin smear. One way to do this is to place a small dab on your index finger tip. Then rub your finger and thumb together to spread it about evenly. Now take the o-ring and rub it between your finger and thumb which will lightly coat the o-ring with grease.

Where possible, and that’s just about always, refit the o-rings with just your fingers to prevent the risk of damaging the o-ring. Try not to use your nails as they can also nick the o-ring rubber. Fit them as soon as you have applied the grease to prevent them picking up any dirt.

let’s get started!

Remove the Buddy Bottle

Whenever you work on your rifle, the first thing to do is to make it safe. Remove the magazine and check that there isn’t a pellet in the breech. Now remove the buddy bottle as follows. If you have a standard Rapid without a Dr Bob’s Quick Fill adaptor, unscrew the bottle one whole turn. This pulls the bottle and its valve away from the regulator push-pin. This close the bottle valve. However, there is still pressure between the bottle and the regulator that needs to be dissipated. This can be done by cocking and dry firing the rifle a couple of times. After the first dry fire shot, the second should sound quieter. If not, unscrew the bottle a quarter of a turn and repeat. Repeat until you can tell that the pressure has been released by the sound of the rifle. The bottle will feel loose once the pressure in the regulator is reduced. Now remove the bottle completely.

Bottle along with Dr Bob’s Quick Fill kit removed

If you have a Dr Bob’s Quick Fill adaptor, it is easier to remove the bottle after taking off the stock (see the next step).

With the quick fill kit you should have an extra foster snap on connector, a set of pins and a bleed screw. Put the snap on connector onto the fill port. Insert one of the pins into the connector and then screw in the bleed screw until the air begins to escape. It’s best to allow the air to escape slowly. When all the air has been released, remove the snap on connector and the pin. Now dry fire the rifle once to release the pressurised air that remains in the block between the regulator and the valve. The buddy bottle along with the quick fill adaptor can now be unscrewed and removed.

If you do not have a quick fill adaptor and you are using the original buddy bottle system, you can remove the buddy bottle valve and replace the o-ring if the bottle is leaking around this o-ring. To do this you must first completely empty the buddy bottle before attempting to remove the valve. This is actually very easy to do. First, remove the buddy bottle from the rifle as described above. Now remove the buddy bottle o-ring and refit the bottle to the rifle. Without the o-ring the air will leak out through the threads as the pin on the regulator pushes open the valve on the buddy bottle. Once it is completely empty, remove the bottle.

To remove the bottle valve, you will need a special tool that fits the Theoben bottle valve. You can buy such a tool from Dr Bob’s Gun Bits or from Best Fittings. I prefer the Dr Bob’s tool so that you can place it in a vice and then place the bottle valve in the tool. Then get a firm grip on the bottle and turn. It’s not really advisable to put the bottle in a vice as compressing it can stress the bottle. You could make up a circular clamp from wood if you have the appropriate tools and clamp the bottle using that in a vice.

Buddy bottles have lifespans just as any other pressure vessel. According to BSA, aluminium bottles without a PI * mark on them should be tested after five years of the date of manufacture. Bottles with the π mark should be tested after ten years. Obviously if you damage the bottle before its lifetime is up, you should replace it immediately. Remember, these buddy bottles are potentially dangerous and can send shrapnel flying through the air if they are not thoroughly respected. The cost of servicing a buddy bottle is similar to buying a new one without the valve. So it makes economic sense to simply replace them when necessary and swap over the valve. There is an interesting article about air gun bottle testing from Hydrotech that is well worth a read albeit a few years old!

Remove the Scope, Mounts and Stock

Removing the scope mounts

This part is straightforward. I am going to assume you know how to remove your scope. The scope mounts are screwed onto the block with allen bolts hidden in the mount itself. You have to remove the scope to gain access to the mount bolts. If you do not need to remove the barrel, you can leave the scope mounts on. But if you are going to fully service and check for corrosion between the barrel and the block, you will need to remove the forward mount as under it is the barrel locating screw.

Removing the stock screw

The stock is held on with a single allen screw found under the stock just forward of the trigger. Unscrew it and then separate the stock from the block. Put the stock in a safe place as you will not be using it for a while and keep the stock screw safe too.

If you wish, you could polish the stock or apply a coat of wax such as CCL Gunstock Wax. It’s probably not a wise idea to use your household polish as the scent will be a dead giveaway should you go hunting.

Remove the Moderator (optional)

Removing the moderator

The moderator is attached to the barrel with a single allen screw found on the underside of the moderator. Remove the screw fully and then slide the moderator off the barrel. It should not be necessary to twist the moderator as you remove it. Doing so is likely to damage the barrel’s finish.

Refit the screw to the moderator so that you don’t lose it. Now put the moderator in a safe place where it will not be damaged.

Remove the Barrel (optional)

Remove the barrel locating screw

To remove the barrel, first remove the barrel locating screw. This screw also forms the forward scope bracket socket so be careful not to damage the internal threads.

Next remove the barrel clamp bolt and slide the barrel out of the block. Refit the barrel clamp screw a few turns and then use it to push one of the barrel clamp wedges out. Remove the screw and wedge. The other wedge can be pushed out using a wooden dowel, chopstick or brass pin punch as required.

Remove the barrel clamp screw, barrel and wedges

Remove the Regulator

Regulator removed from the block

Place the block in a sturdy vice with soft jaws and a rag to protect the finish of the block. Using an open-ended 21mm spanner, remove the regulator from the block. The regulator is normally “nipped” and so once you break the lock, it will be loose. If you still have the barrel attached, you may wish to protect it with a rag in case you slip.

Usually, I remove the bolt for this step and put it back on in later stages. Removing the bolt allows you to place more of the block into a vice for a secure grip.

Remove the Blanking Plug

Remove the blanking plug

Theoben often fitted different types of blanking plug. The one fitted to my rifle was originally a brass plug with an o-ring. Being brass, the allen socket did not last very long and I had to replace it with a steel plug and Dowty seal (a washer with bonded rubber seal). I believe Theoben also used similar steel plugs and Dowty seals as well at some stage during production.

Remove the Valve

First remove the valve cap. Then, remove the valve spring and stem. Often the valve spring is firmly attached to the stem and both will come out together. If the valve stem doesn’t come out, try with a pair of pliers but do not force it. The valve stem should come out really easily with no effort. If it still will not come out, it can be removed along with the valve body as follows. I do not advocate pushing the stem or valve out from the hammer side with improvised tools as there is a risk of damaging the valve stem guide and the stem itself.

Make sure you remove the valve guide screw which is on the right hand side of the block. Failure to remove this screw will cause the valve body to be damaged and a new one will need to be sourced. Next, refit the bolt handle and cock the rifle. Cover the valve port with your hand or a cloth. Fire the rifle and catch the valve body and stem in the cloth or your hand. It is important not to let them fly across your workshop or they may be irreversibly damaged!

Valve cap, spring and stem removal

Depending on the type of valve that Theoben fitted, yours will either have one or two main o-rings and a smaller o-ring around the valve stem guide. The smaller o-ring slides off but replacing it is a little tricky as it needs to be stretched over the valve stem guide. Don’t forget to lubricate the o-ring with a tiny smear of Molykote 33 as I described previously.

Remove the larger valve body o-ring(s) using an o-ring picker. Be careful not to score the o-ring groove or it may cause a leak on reassembly. If you score it, you may need to source a new valve body.

The valve stem is fitted with one o-ring. Again be careful when removing it. Some stems have two grooves. Only one o-ring should be fitted to the slot nearest the spring side of the stem.

Valve body and stem o-ring removal

Remove the Hammer

Unscrew and remove the hammer adjustment screw cap. Then remove the hammer spring along with its spring guide. There may be some preload washers in the end of the cap to watch out for as well. Place your hand over the hammer cavity and tilt the body up so that the hammer will slide out into your hand. Inside the hammer is a washer, weight and spring. This is the debounce weight that helps to reduce the hammer bouncing on the valve stem. In turn this prevents wasted air and thus improves the shot count. The washer acts as a bulkhead to prevent the debounce weight from falling out and into the hammer spring. You may need to depress the sear lever arm through the slot in the side of the block in order to remove the hammer. You can do this with a thin allen key. Alternatively you can follow the procedure to strip down the trigger as described in the next step.

Hammer assembly and removal

Remove the Trigger

Theoben fitted a number of variations of trigger designs to the Rapid MKII. I will cover disassembly and servicing of the early MKII Sporter trigger design as that is the type fitted to my Rapid.

Remove the safety catch and second stage adjuster assembly

Remove the two allen screws that hold the safety catch and second stage adjuster screw assembly in place. The safety catch merely prevents the trigger from being pulled and it is very easy to knock the safety off. Personally I don’t use it and never rely on any safety. I’d rather be sure the rifle is discharged than rely on a safety catch.

There is a spring-loaded ball bearing in the safety catch mechanism in a vain attempt to help hold the safety in place when set. On my rifle, this barely has any effect. You can remove the safety by sliding it out of the plate. Just be careful not to lose the ball bearing as it will shoot across your workshop. Here’s a tip to prevent this from happening… place the assembly in a sealed ziplock bag and then remove the catch from the plate. There’s no way that ball bearing will escape from the bag!

Safety catch and second stage adjustment disassembly

The second stage adjuster simply unscrews from the plate. Inside the adjuster is another grub screw and spring. This inner grub screw adjusts the tension of the spring which acts on a captive ball bearing and in turn sets the second stage pull weight. The larger outer grub screw sets the second stage detent point. I will fully describe how to adjust the trigger later in the article.

Push the trigger pivot pin out with a brass pin punch

Next remove the trigger by pushing out the trigger pivot pin. You can use the end of an allen key for this task or a matchstick. The pin should slide out easily without needing to use a hammer and punch. If it doesn’t come out easily, try pushing it out from the other side instead. Failing that, a light tap with a small hammer and brass parallel pin punch should do the trick. Remove the trigger blade along with the lever spring. Don’t worry, this one isn’t going to shoot out across the workshop.

Remove the allen screw holding the sear strut adjustment screw and spring in place. Again nothing is going to fly out across the workshop. With the sear strut adjustment assembly removed, the sear strut can be pulled out as there is nothing left to hold it in place.

Remove the sear strut tension adjustment assembly

The last stage of the trigger disassembly is to remove the sear lever. Push out the sear lever pivot pin. Again no effort should be required as these pins are not tapered. Once out, either pull out the sear lever and its two bushes from the trigger cavity or push it out through the hammer tube.

Remove the sear lever pin, lever and bushes

Remove the Bolt

To remove the bolt, you must remove the hammer assembly as described earlier. You can remove the bolt without removing the trigger but you will need to remove the second stage adjuster and safety catch assembly in order to gain access to the cocking dog.

Hexagonal cocking “dog”

If you look through the trigger cavity and up to the bolt, you will see the cocking “dog” towards the front of the bolt. It is hexagonal in shape and is screwed into a slot in the bolt using an allen bolt. Remove the allen bolt and withdraw the cocking dog. Be careful though, make sure your allen key fits well as this is a small allen bolt.

Inspect the dog for wear. It has six sides, so if one side is worn too much, rotate the dog when you refit it for a fresh face.

Now that the cocking dog is removed, remove the bolt handle and then slide the bolt out from the rear of the block. The bolt gate is held on by two allen screws. Remove the allen screws and the gate. Then check for corrosion under the gate. The sole purpose of the gate is to protect the soft aluminium of the block from wear whilst repeatedly cocking the rifle.

Cocking dog and bolt removed

Bolt Disassembly

Remove the locking grub screw from the side of the bolt. Then remove the probe adjustment grub screw from the rear of the bolt. Behind the adjustment grub screw is a rod which should slide out if you tip the bolt upwards.

Remove the locking grub screw, probe adjustment grub screw and rod

Remove the circlip that holds the probe in place. Then withdraw the probe, spring, belleville washer and bolt stem.

Remove the circlip and probe components

Regulator Testing

Before you jump in feet first and strip the regulator, you will need a regulator tester in order to test it and set it to the correct pressure after reassembly. In fact you can use the regulator tester to check your regulator is or isn’t functioning correctly before you determine whether you need to strip it down to replace any seals.

There are two types of regulator tester that I am aware of. There is the Dr Bob’s one that attaches to your dive cylinder and utilises the cylinder’s pressure gauge. Then there is one from Best Fittings.

With the Dr Bob’s tester, the charging cylinder acts as a plug. The valve is kept shut so you are not using the air from the charging cylinder. The gauge from the cylinder or your filling adaptor is used to read the pressure setting of the regulator. The second type, from Best Fittings, is effectively a tube with one end blanked off with. It is fitted with a bleed screw and an integrated pressure gauge.

Removing the nylon lock-nut

Both work the same manner. One end is effectively capped off with a gauge to measure the pressure of what would be in the block. The regulator is screwed into the other end and the buddy bottle is screwed into the regulator just as it would be on the rifle. If you have bought a quick fill adaptor as I have, this could be where you come undone. If you have used your original bottle, you can still run the test but you will need to fill your bottle from empty. If you only have one charging cylinder you will not be able to fill the buddy bottle unless you use the Best Fittings tester. What’s worse is the each time you need to adjust the regulator, you will have to empty the buddy bottle, adjust, refill and test again. This is very wasteful of the air in your charging cylinder.

Depending on the calibre of your Rapid MKII, the factory set pressure for the regulator should be 1,700 psi (117 bar) for .177 rifles, 1,450 psi (100 bar) for .20 rifles and 1,200 psi (82 bar) for .22 rifles.

Using the tester is straightforward. Close off all bleed valves. Attach the tester to your dive cylinder (if required) using an adaptor or your filling hose adaptors. Screw in the regulator and then screw on the buddy bottle. The pressure of the regulator can be read directly off the gauge.

To remove the regulator from the test rig, unscrew the buddy bottle one turn whilst holding the regulator with your 21mm spanner. Now open the bleed screw. If the air keeps flowing, close the bleed screw and unscrew the bottle another quarter of a turn and repeat. It’s just like removing the bottle from the rifle.

Not only should you check the pressure setting of the regulator, but you should check that it returns to pressure quickly and without any creep whilst simulating firing of the rifle. To simulate firing of the rifle, open and close a bleed valve quickly and watch the pressure gauge. If it returns quickly to the set pressure then your regulator is working correctly. If it returns slowly, or stops short and then creeps up, the chances are that either the o-ring under the pin disc is faulty, or the disc is loose, or the nose seal of the regulator needs to be resurfaced.

Regulator Disassembly

The regulator can be disassembled quite easily. Depending on the type you may need some circlip pliers. The alternative regulator design has a threaded collar that holds the regulator core in place. This can be unscrewed to separate the core from the regulator body.

Removal of the circlip

First of all you need to remove the lock-nut from the top of the regulator core. You will need to hold the core still whilst removing the nut with a 10mm ring spanner. Original Theoben regulators had an allen key socket in the top of the core. But my regulator has been modified and now has a slot for a screwdriver instead. Some later modified regulators may have a torx socket.

Once you’ve removed the lock-nut, remove the belleville washers. Make a note of how they are fitted as this is particularly important for the correct functioning of the regulator. It’s a good idea to take some photos at this stage.

The belleville washers are cone-shaped and act as a spring. The spring tension pulls the regulator core away from the sealing face at the nose of the regulator. This allows high pressure air through the regulator and into the block. As the pressure in the block increases, the force of the air against the rear of the regulator core pushes the core back into the regulator and closes the seal. The pressure at which the nose seal closes is set by adjusting the tension on the belleville washers. The higher the tension, the higher the pressure needed to overcome the force of the spring.

Brass disc incorporating the bottle valve push-pin

The belleville washers on my rifle are fitted in pairs in this ()()() configuration. Other configurations have been observed so it is important to note how yours was assembled. Now is the time you wished you had taken that photograph.

Once the nut and belleville washers have been removed, you can unscrew the collar to remove the regulator core or use a pair of circlip pliers to remove the circlip should your regulator be of that design. With the circlip removed, the regulator core can be withdrawn from the body of the regulator.

The individual components of the Rapid regulator

In the bottle end of the regulator body is a brass disc. This disc incorporates the bottle valve push-pin that opens the valve of the buddy bottle as the bottle is screwed into the regulator.

Under the brass disc is an o-ring that may need to be replaced. The disc can be removed with a pair of sturdy circlip pliers. Sometimes the brass disc can become stuck in place and you might think you will snap the pliers trying to remove it. I needed to hold the regulator body in a vice wrapped in a bicycle inner tube for grip and protection. You could fashion a removal tool out of a deep hex socket using a dremel or bench grinder. This would allow you to use a longer lever on the socket to make it easier to release the lock.

In all, there are four regulator o-rings. Two on the core, one on the pin disc and one on the regulator body. As a matter of course all should be replaced. There is also a delrin rod seal on the nose of the regulator core. Replacing this requires a lathe to remove the old rod and to furnish a new one. You may be able to reface the nose seal by placing it in a power drill held in a vice and then using very fine wet and dry on a block of wood to lightly remove any impressions. The sealing face on the brass disc can also be polished in a similar manner. Alternatively, drop me a message and I will put you in contact with a very capable guy who services and upgrades Theoben regulators for a very reasonable fee.

strip down complete!

That’s the Rapid fully stripped down. Let’s start putting it back together. It is imperative to have a clean workspace to ensure that o-rings and high pressure air parts are not contaminated with dust, lint or hair. Any debris on the o-rings and parts can cause leaks and any debris that finds its way onto the valve faces can cause permanent damage until they are refaced or replaced. I usually use a sheet of kitchen roll to place cleaned o-rings and parts on. Alternatively you could use some kitchen foil. Always assemble greased o-rings immediately and never put them down as they will pick up debris.

Clean the Block

Thoroughly clean the block. Remove any traces of grease or oil. When doing so, be careful not to score the valve chamber or you will cause irreversible leaks and it will be game over for the rifle. The block can be cleaned with a solvent if necessary. A carburetor or brake cleaner can be used or even isopropyl alcohol. Dry the block thoroughly after cleaning. I use air from a compressor to blow out all the drillings and cavities. Try not to use towels as they can leave lint behind that can cause leaks. You can use old toothbrushes but test then with the solvent first to ensure they do not melt as you do not want the block contaminated. I can’t stress enough how important cleanliness is during reassembly of PCP air guns.

Valve Assembly

Start with the valve chamber plug. Remove its o-ring and clean the plug. Fit a new, lightly greased o-ring and place it to one side.

Fit the new o-ring to the middle slot

Remove the o-ring from the valve stem and clean the stem. Fit a new, lightly greased o-ring and put it to one side also. The stem may have two o-ring slots. Only one should be fitted to the groove in the centre of the stem.

Remove the o-rings from the regulator body. Be careful not to score the o-ring grooves of the plastic valve body as this can cause leaks. Clean the valve body. If you have a plastic valve body, wipe it clean without solvents. Use a compressor to blow any debris or lint away. Fit new, lightly greased o-rings.

Block with locating screw facing up

Place the block in front of you on its side with the valve locating screw hole facing upwards. Now put the valve stem into the valve body, then align the valve body with the valve chamber of the block. The valve body has two holes on the side. One is drilled all the way through and is the transfer port. This should be orientated to face the barrel opening. The other does not go all the way through and is the valve screw locating slot. The screw locates into this slot and prevents the valve from rotating during use. Thus it keeps the valve port aligned with the barrel port. Align the valve so the locating slot facing upwards and the transfer port towards the barrel. Now push the valve body into the block using the stem. Look through the locating screw hole to ensure the slot is aligned correctly. You should see the slot come into view through the screw hole.

Two valves: one with brass plate, one without

Some valves were fitted with a small brass plate in the locating slot. This makes it much easier to check the valve is correctly aligned by looking for the brass plate through the locating screw hole. If your valve does not have the brass plate, you could paint the slot with a small amount of white paint, tippex or nail varnish. Personally, I don’t paint mine. If you are careful and take your time you will see the slot through the locating screw hole as you insert the valve.

Back off the locating screw by a quarter of a turn

Once the valve is correctly located, screw in the locating screw. Do not tighten it down! When you feel the screw make contact with the valve body, back it off a quarter of a turn to allow the valve to move slightly in the chamber.

Now screw the valve cap into the block. There is no need to tighten this down as it is the o-ring that makes the seal not the cap itself. The valve cap will not work loose as it is held in place by the regulator body.

Regulator Assembly

Once you have removed all the o-rings, clean the regulator thoroughly. I use kitchen roll to wipe clean but also make up sponges on sticks to get into the o-ring grooves. Reassembly is the reverse of disassembly. I always fit new, cleaned and greased o-rings. They’re so cheap that there is no point in risking a leak.

The pin disc does not need to be done up tight. Just nip it. It’s the o-ring that will make the seal not the disc itself. As the o-ring is exposed to pressure, it will be squeezed into place to form a seal. This is why the o-rings are greased as it helps to manipulate the o-ring into place. Thus only the thinnest smear of grease is required.

Refitting the regulator core and circlip

Next fit new o-rings to the regulator core and insert it into the regulator body. There is a trick to fitting the circlip. Pull the core out of the regulator so that the circlip groove on the core is just outside the regulator body. Slip the circlip down onto the shoulder of the core and squeeze the eyes of the clip together with the circlip pliers. Push both the circlip and the core into the regulator together. Ensure the circlip is firmly located under the lip of the regulator body.

Refit the belleville washers in the same configuration as they were originally fitted and replace the lock-nut. The lock-nut is the means to adjust the pressure of the regulator. Therefore it is not a nut to be tightened down! Using a 10mm ring spanner and, in my case, a screwdriver to hold the core still, screw the nut down until the top is level with the top of the regulator core. This will give you a starting point from where you can fine tune the regulator pressure.

Checking the regulator pressure

Install the regulator in the regulator pressure test rig and check the reading on the pressure gauge. You need at least 100 bar in your buddy bottle. Any less and you will may not be able to set the regulator pressure correctly. The Dr Bob’s regulator tester uses a larger sized o-ring between the regulator and the regulator tester. You will need to fit this instead of the o-ring that will be used when the regulator is fitted to the rifle block.

Depending on the calibre of your Rapid MKII, the factory set pressure for the regulator should be 1,700 psi (117 bar) for .177 rifles, 1,450 psi (100 bar) for .20 rifles and 1,200 psi (82 bar) for .22 rifles.

Unfortunately, to adjust the regulator you will need to remove it from the test rig. To remove the regulator from the test rig, unscrew the buddy bottle one turn and then open the bleed screw. It’s just like removing the bottle from the rifle. If the air keeps flowing, close the bleed screw and unscrew the bottle another quarter of a turn and repeat until only the air in the regulator is released and the buddy bottle is loose. Now you can remove the buddy bottle and then the regulator.

If the regulator needs adjusting, place the ring spanner over the nut and hold the core still with a screwdriver, allen key or torx bit as necessary. To lower the pressure, turn the nut anti-clockwise. To increase the pressure, turn it clockwise. I adjust the nut on the regulator 1/4 or 1/8th of a turn at a time and retest.

Refit the correct regulator body o-ring

Don’t forget to fit the correct o-ring to the regulator body as the regulator tester o-ring will not fit when attached to the rifle block.

Fit the regulator to the block taking care not to cross the threads. The regulator should screw in by hand and only require a spanner to nip it tight. If you do not nip it tight, the regulator will come loose when you remove the buddy bottle for filling which is never a good thing to have happen. You will need to use a vice again to nip the regulator tight. Make sure you use soft jaws and a cloth to protect the finish of the block. The regulator is only going to turn maybe 1/8th of a turn or less from finger tight. So don’t over do it!

Blanking Plug

Remove the blanking plug

Refit the blanking plug. Use a new o-ring or Dowty seal as appropriate and lubricate with Molykote 33. Again, there is no need to over tighten the blanking plug. Just a slight nip will hold it in place.

You might wonder what the point of the blanking plug is. In order to make a passage between the regulator and valve chambers, a hole had to be drilled between the two. The only way to do this is to drill through the block from the bottom. The hole was then tapped to allow a blanking plug to seal the external hole.

Bolt Assembly

Clean the bolt and its components of any grease. Then place a smear of molybdenum grease on the inside of the bolt where the probe is housed. Just a smear, no need to over do it. Insert the bolt stem, then the belleville washer cup side up followed by the spring and then the bolt probe. Fix it in place with the circlip.

Loctite 222 low strength thread lock solution

Replace the adjustment rod. You can put a smear of molybdenum grease on the rod if you like but I tend not to bother. Then screw in the long adjustment grub screw until it is flush with the base of the bolt body. Theoben’s instructions recommended placing a line of Loctite onto the adjustment grub screw before assembly. But those instructions also omit the locking grub screw. If your bolt does not have the locking grub screw then use some pink Loctite (222) or similar low strength thread lock solution. Otherwise, the locking grub screw will be sufficient without the need to apply Loctite.

Lightly coat the bolt with molybdenum grease and slide it into the block. Then refit the cocking dog and screw through the trigger cavity. You may wish to use a small drop of pink Loctite (222) on the cocking dog screw threads. Again, as with all the screws on the Rapid, there is no need to tighten this too much. Just a slight nip. The Loctite will do the rest.

Slide the bolt assembly into the block and refit the cocking gate

Fit the cocking gate. You can use pink low strength Loctite on these screws if you like although I don’t bother. Fit the bolt handle. Again, low strength Loctite can be used if you wish.

Adjustment of the bolt probe will be discussed later as part of the final setup of the rifle.

Hammer Assembly

The hammer and components should be assembled dry. By dry I mean without any grease or oil at all! Any lubricant will cause variable drag on the hammer which in turn will cause variations in the amount of air delivered to the pellet. This would ultimately affect the performance and grouping ability of the rifle. So, suffice to say, thoroughly clean the hammer and its components!

Polishing wheels and compounds

If you wish to try to improve the performance of the Rapid, and by that I mean lower the variance of feet per second from shot to shot, then you can polish the hammer so that it slides down the hammer cavity with less drag. The easiest way to do this is to use a polishing wheel on a drill that is held in a vice or workmate. With the drill power switch locked in the on position, polish the thicker faces of the hammer that contact the sides of the hammer tube. But be careful not to round off the edges or you may find the sear lever no longer holds the hammer back!

Slide the hammer into the block. If you like, you can polish the debounce weight to help it move with less friction inside the hammer. Just use a cloth and some Autosol polish. Now drop the debounce weight with the spring attached into the hammer spring end first. Be sure to check that the debounce weight and spring drop into the debounce cavity. Next drop in the washer and make sure it is lying flat. The washer prevents the debounce weight from falling back into the hammer spring.

Now fit the hammer spring with the spring guide at the rear. Finally screw in the hammer adjuster cap. Screw this in all the way and finger tight only. This is an adjuster, not a fixing bolt and thus it should be “loose”.

The hammer assembly and order of components

Trigger Assembly

Clean all the parts of the trigger mechanism removing all traces of the old grease and oil.

Add a drop of Abbey SM50 oil to the sear lever bushes and fit them to the sear lever arm. Now slide the sear lever with the bushes into the trigger cavity and align the sear lever pivot hole with the hole in the block. Put a drop of oil on the pivot pin and slide it into place. It should not need hammering in as it is not tapered.

Fitting the sear lever arm and bushes to the block
Fit the sear strut into the sear strut cavity

Now fit the sear strut. You can polish the outside of this if you like. Do this by hand with some Autosol. Do not use any power tools. You only want to polish the sliding faces to help improve the mechanism. Alternatively you could apply some graphite powder but my concern with this is that it can end up in places where you don’t want it and cause more headaches than necessary. Polish the sear strut and assemble it into the sear strut cavity dry. Make sure it locates over the sear lever arm.

Fit the sear strut tension adjuster to the block

Fit the sear strut tension spring into the sear strut. Then with the sear strut tension adjustment screw fitted to the sear strut retaining plate, screw the plate into position. You may find it easier to fit the spring to the adjustment screw first. There’s no need for any Loctite here as the spring tension will hold everything in place.

You may wish to polish the contact point where the spring-loaded ball bearing second stage adjuster makes contact with the trigger lever. Mine was quite rough and so I polished it with a dremel polishing pad and compound to remove any roughness of this part of the trigger pull.

Fit the trigger blade and return spring

Refit the trigger and return spring. Place a smear of molybdenum grease on the sear contact points and on the contact point with the spring-loaded ball bearing. Use a drop of Abbey SM50 oil on the trigger pivot pin and insert it into the block and trigger blade. Again, no force should be necessary as this is not a tapered pin.

Reassemble the second stage adjuster. Add a drop of oil into the second stage adjuster, then fit the spring and inner grub screw. Screw it into the safety catch plate then assemble the safety catch with a smear of molybdenum grease on the sliding edges of the plate. Fit the safety catch and second stage adjuster assembly to the rifle taking care to ensure the trigger return spring is located under the plate but not trapped between the plate and the block. You may find holding the spring down with a thin allen key placed along the side of the trigger will help keep it in place. Don’t forget to remove the allen key!

Refit the safety catch and second stage adjuster assembly

Trigger Adjustment

The trigger on this rifle is the early Theoben Sporter trigger with sear lever arm. Some models of this trigger did not have the sear lever arm and relied solely on the sear strut to hold the hammer back. Also, some models with the sear lever arm used two sear lever springs. One for the lever itself and one for the sear lever strut. The Sporter trigger on this particular rifle has only one spring which is fitted to the sear lever strut.

I found the following technique the simplest to set up the trigger. First, back out the second stage detent adjuster. This is the large grub screw that is in front of the trigger. This ensures that the trigger has enough movement to begin with.

Back out the second stage detent adjuster (large grub screw) in front of the trigger and adjust the sear lever strut tension using the grub screw behind the trigger

Now screw in the sear strut spring tension screw. This is the smaller grub screw that is behind the trigger. Cock the rifle and pull the trigger back. The hammer should not release. Whilst holding the trigger back, unscrew (anticlockwise) the sear strut spring tension screw until the hammer is released. Now unscrew it another half turn. This should give enough tension in the spring to hold the sear lever up against the force of the hammer that is trying to force its way past the sear.

Setting the second stage detent

Now screw the second stage adjuster in until it just makes contact with the trigger blade. Then unscrew it one turn. Cock the rifle then gently pull the trigger. You should be able to feel the trigger stop when it reaches the ball bearing in the second stage adjuster. This is the second stage detent. If the hammer was released and you didn’t feel the second stage detent, then turn the adjuster in a quarter or an eighth of a turn and repeat. The idea is to set the second stage detent so that you can feel a positive second stage detent but also so that only a slight pull after the second stage detent releases the hammer. If it requires is a long pull after the second stage detent to release the hammer, then you need to unscrew the second stage adjuster a quarter or an eighth of a turn at a time to set it just right.

Another way to think of this is that the trigger will always move the same overall distance. Turning the second stage adjuster merely moves the point where the second stage detent is felt. After which there should be practically no travel required to release the hammer.

Adjusting the second stage pull weight

Finally set the second stage pull weight. This is achieved by turning the smaller grub screw that is inside the second stage adjuster. Turning the screw clockwise, or inwards, increases the pull weight required to release the second stage. If you screw this in too far, the ball bearing will not be able to move and it may not be possible to release the hammer. If it is set too loose, you may not be able to feel the second stage or the hammer might be released at the slightest twitch of the finger.

Do make sure you perform the “final safety checks” later in this article after adjusting the trigger.

Barrel Assembly

Clean the barrel clamp wedges then apply some molybdenum grease to help them slide within the block easier. The grease will also help prevent moisture ingress and limit corrosion. Insert the clamps into the block wedge side lowest and towards the inside. Put a smear of molybdenum grease on the thread of the bolt and screw it in just a few turns.

Napier airgun pull through kit

If you wish to clean the rifling, now is a good time to do it although you can do it just as easily with it attached to the rifle. I use the Napier Airgun Pull Through kit. You could make your own pull through if you wish.

Apply some molybdenum grease to the outside section of the barrel which fits inside the block. Again this will help to prevent moisture ingress and limit corrosion. Insert the barrel into the block with the transfer port facing the valve and the barrel alignment hole facing the hole in the top of the block where the scope mount hole is located. Some barrels may have the calibre stamped on the top which can help with orientation of the barrel. But to be certain, the transfer port is drilled all the way through to the center of the barrel whereas the barrel locating hole does not go through to the center of the barrel. It really isn’t wise to get this the wrong way around!

Insert the barrel then fit the barrel locating screw

Insert the barrel locating screw into the top of the block and screw it in lightly. Back it off a turn once you feel it touch the barrel. You should be able to move the barrel back and forth a small amount so that you can correctly set its position to allow the magazine to fit just right. The purpose of the barrel locating screw is to prevent the barrel rotating out of alignment during the barrel adjustment procedure.

Use a piece of paper to help set the barrel position

To set the barrel position, start with the barrel flush with the magazine slot or just recessed into the block. Pull the bolt back and lock it in position. Now fit the magazine with a sheet of paper or post-it note between the magazine and the block. Push the barrel back towards the magazine until it meets the magazine. There’s no need to force it. It just needs to touch the magazine. Now tighten the barrel clamp bolt to hold the barrel securely in place. Finally tighten the barrel locating screw. Just a little nip, not too much as it’s the barrel clamp wedges and bolt that holds the barrel in place. You want to nip the locating screw so that it doesn’t come out with the scope mount screw the next time you remove the scope mount.

Adjust the Bolt

It is important to set the bolt adjustment after the barrel position has been set otherwise it will be incorrect after adjusting the barrel.

Adjusting the bolt probe tension

Loosen the locking grub screw on the side of the bolt. With the bolt locked forward, screw in the adjustment grub screw from the rear of the bolt. Do this until it starts to feel harder to turn. Then screw it in another 1/8th to 1/4 of a turn more. Now check the feel of the lock down. If it is too light, air may escape from the breech. If it is too heavy, it will be difficult to lock down. There is a final check which can be performed when the rifle is fully assembled and pressurised (see next step).

Refit the Stock

If you have a Dr Bob’s or similar quick fill kit, this is the time to refit it. Fit new o-rings lightly greased with Molykote 33. Align the gauge and fill port. This is a good time to add some air and check for leaks. I put about 50 to 100 bar into the buddy bottle. If you have a leak and fill to 200 bar, you’re just going to waste air in order to strip the rifle down to fix the leak.

Refit the quick fill adaptor and buddy bottle, then check for leaks

Dilute some washing up liquid in some water and lightly apply the solution around the joints of the bottle, the regulator, the valve cap and the plug that’s in the bottom of the block. Don’t over do it. You don’t really want to get the solution in the hammer, trigger or bolt. Now look for bubbles. Bubbles indicate a leak and the location of the leak. Also, wet your finger with the solution and place it over the muzzle of the barrel with the bolt forwards. It will be obvious if there is a leak here as the air will be fizzing out around your finger or a slow bubble will form. If there is a slow bubble, it could be that there isn’t enough pressure in the rifle to force the valve closed fully. Just be sure there is about 100 bar in the rifle.

Refit the stock

If there are no leaks, fit the stock and stock screw. Remember, the block is aluminium so do not over tighten the stock screw or you may strip the threads. As with all the screws, just a light nip is required.

If you have a standard buddy bottle rather than a quick fill system, now is the time to fit the buddy bottle. Make sure you’ve filled it first though. If you wish, you could try to fit the buddy bottle before fitting the stock. Then you can check for leaks before fitting the stock. You may have to remove the buddy bottle in order to fit the stock. With a quick fill kit, it is possible to remove and refit the stock with the bottle still attached.

Refit the Moderator and Scope Mounts

Refit the moderator

Now the last few items. Refit the moderator. Slide it over the barrel with the locking screw aligned to what will be the underside of the barrel. Once the end stop reaches the muzzle, tighten the locking screw. As always, just a nip.

Now refit the lower half of the scope mounts. Don’t fit the scope at this stage as you need access to the magazine slot and probe for the final leak check.

Probe Leak Check

Remember to tighten the probe adjustment locking grub screw!

Cock the rifle and lock the bolt forward without the magazine. Place a piece of tissue over the magazine slot and fire the rifle. If the probe is adjusted too lightly, a puff of air will escape from the breech and the tissue will blow off. Readjust the probe as necessary to stop the leak but also so that the bolt is not too heavy to lock down.

Finally, remember to tighten the probe adjustment locking grub screw!

Final Safety Checks

There are a couple of safety tests worth completing after working on the trigger. First the bump test. Cock the rifle and lock the bolt forward. Obviously the rifle should not be loaded. Keep the muzzle pointed away from you at all times. Hold the rifle by the stock vertically with the muzzle upwards. Now “bump” the bottom of the stock onto the workbench or floor. You don’t need to whack it down. Just a bump. If the trigger is set too light, the bump will cause it to release prematurely.

The second check is to allow the bolt to slip. Cock the bolt back and lock it down in the rear position. Now flip to bolt handle up. The rifle should not fire. Again, if the bolt flies forward, the trigger has been set too light.

If the trigger is set too light, screw in the sear strut tension screw 1/8th of a turn and retest.

Check the Power

Last but by far not least you should check the power of your rifle using a chronoscope. Not a catalogue or a block of wood! This is not only to remain legal in some countries but to ensure that the power, or velocity of pellets, do not vary wildly. If it isn’t reasonably consistent, then the fall of the pellet will also vary. Which is something you want to avoid in order to maintain precision.

Jewellery scales

You can weigh your pellets if you wish and then shoot a consistent batch of pellets that are of the same weight. This will give you the most accurate reading. I tend to do this to see just how well my rifles are performing. You should be able to achieve a velocity variation of under 10 feet per second if not less than 5.

I use a set of reasonably cheap jewelry scales from eBay that measure in grains and have calibration weights. It is important that the scale can measure in grains as that forms part of the power calculation.

The Air Chrony MK3

I use the Air Chrony MK3 from air-chrony.com. It uses infrared emitters and sensors and therefore works in most light conditions. If used outside, you may need to shield it from direct sunlight to prevent erroneous readings. It’s perfect for use indoors under any type of domestic lighting without any additional accessories.

It is strongly advisable to always ensure you use a safe backstop or a pellet trap. I also strongly recommend wearing safety goggles just in case you accidentally strike the chronograph or in case of ricochets.

Use a shooting rest to align the rifle with the chronograph

It’s also wise to mount the rifle in a shooting rest with this type of chronograph. It allows you to align the rifle carefully with the chronograph and minimises the risk of shooting it!

I always shoot at least 10 pellet strings with at least 150 bar of air in the rifle. This ensures it is “on reg”. Firing one or two shots will not tell you if the rifle is consistent.

The Air Chrony MK3 will record the velocity of each shot and calculate all the statistics for you. You can also connect the Air Chrony MK3 to a laptop or computer and it will again record each shot, calculate the statistics and save the information to a file for future reference.

If the power is too high, or too low, it can be adjusted by turning the hammer adjustment screw in or out. Try adjusting it by half a turn at a time for groups of five or so shots until you have it were you prefer it. For me, I set my rifles no higher than 11.5 ft/lbs. 12 ft/lbs is the legal limit in the UK without a Firearms Certificate. This ensures there is enough margin of error to remain within the law.

The results from my testing are shown below and I think you will agree that the maximum velocity spread of just under 5 fps is remarkable for such a simple PCP design compared to the complex designs of more recent air rifles.

Power test results

ta da! all done!

That’s the Rapid MKII fully serviced. Now all you need to do is refit your scope, lead the barrel (see this article on leading your barrel) and enjoy the rewards of your efforts! Don’t forget to make sure the scope is levelled correctly by following my scope cant article.

Of course you will also need to zero the scope now that it has been refitted. I will write an article soon telling you all about how to zero your scope and how to set it up using “chairgun pro”.

Oh, one last thing… give the rifle a wipe over with a lightly oiled cloth to remove all those acidic fingerprints and provide a protective coating of oil. I use Napier VP90 sprayed onto a cloth, allow the solvent to evaporate and then wipe over.

Until next time, happy shooting!

Jimmie Dee 

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