Riflescope Setup – Part 3: Rifle and Scope Calibration

Quite often you hear shooters ask “What’s the best range to zero my scope at?”. It’s a perfectly valid question and yet the response can be varied from 25 to 35 yards or “the range at which you will shoot at the most”. Whilst there is nothing wrong with any of those answers, they won’t necessarily help the shooter get the most from their rifle and scope combination and there’s certainly never any mention of magnification.

So what is scope and rifle calibration? Well, it’s the process of determining the best zero range and magnification to make the best use of your scope.

When I say scope, I’m referring to one that has a mil-dot reticle or similar. Not a centre point or simple crosshair scope. But one that has lines or dots marked along the vertical axis of the reticle.

I’m not going to talk about mil-dots per se in this article. For short-range air rifle use out to say 60 yards or so, they are primarily used as aiming points. They can be used for range estimation which is taking the sport to a whole new level. For now, let’s concentrate on using them as aiming points.

first focal plane vs second focal plane

The scope of this article (did you see what I did there) is also limited to second focal plane scopes as opposed to first focal plane scopes. Most scopes that are available today and the most common that are used on air rifles are second focal plane scopes. With these, as you adjust the reticle, the size of the reticle does not change with respect to the image seen through the scope. This means that the distance of the mil-dots at which you aim changes as you change the zoom. Whereas with a first focal plane scope, the reticle also increases or decreases in size as you adjust the image size using the magnification ring. Thus with first focal plane scopes, the distance for each mil-dot aim point does not change when the magnification is adjusted. Suffice to say, first focal plane scopes tend to be more expensive than second focal plane scopes although the price difference is beginning to reduce.  

theory

The process I am about to describe is ideally suited for .22 calibre air rifles. It is also applicable to other calibres as well although the results are entirely different as I shall endeavour to explain.

Why .22 calibre in particular? Well, .22 compared to say .177 has a much more pronounced parabolic trajectory than .177 over similar distances. This means that for a given distance from the shooter, the impact point of the pellet will be much lower than that of a smaller calibre pellet such as .177.

Did I lose you at “parabolic trajectory”? A parabolic trajectory is a curved path an object follows when it is launched into the air as gravity pulls it back down. Imagine yourself in a game of baseball. The pitcher throws the ball and you hit it high. Unless you are Superman, that ball is going to come back down to earth soon. As it leaves your bat, gravity instantly takes effect and tries to pull the ball down, forcing the ball along a curved, parabolic path.

When I calibrate a scope for a .22 rifle, my goal is to achieve maximum use of the reticle over a range that extends to 55 or 60 yards with each mil-dot representing a 5-yard increment. Those 5-yard increments make it much easier to remember the distance that each mil-dot represents as you count up or down from the centre point.

To do this I use a computer program called Chairgun Pro. This is a free application from Hawke, thanks guys, and you can use it for any mil-dot scope. Not just those made by Hawke. To successfully calibrate the scope and rifle combination, you need to know some basic details about your rifle and scope.

First, you need to know the muzzle velocity of the pellets or the power of your rifle. This means that you need to use a chronograph to measure the velocity of a number of pellets shot from your rifle. This needs to be done near the muzzle rather than downrange. Also, there’s no need to weigh the pellets for this exercise. Just shoot ten or more pellets straight from the tin through the chronograph and note the velocity of each. Then calculate the average velocity from your test. Ten is enough to give sufficient results. But what is important is that you select the pellet that provides the tightest group for your rifle. If you decide to shoot with a different pellet at some time in the future, you will need to recalibrate your rifle as a change in weight and the ballistic coefficient can affect the flight path of the pellet.

Ballistic coefficient??? Stick with me, you might learn a thing or two. Ballistic coefficient is a measure of how well a pellet cuts through the air. Each pellet design is different and so is its ballistic coefficient. For example, a domed head pellet will cut through the air better than a flat head pellet. Luckily for us, Hawke has provided the ballistic coefficient for many common pellets. All you need to do is select the pellet in the application. If your pellet isn’t listed, check with the manufacturer or you can calculate it by measuring the velocity of the pellet near the rifle and then again at the target. Check this page for a calculator that will help should you need it.

You also need to know the pellet weight. If you’ve selected your pellet in Chairgun Pro, it will have already provided the correct “tin” weight. If it hasn’t, read it from the tin or check with the manufacturer. It’s worth double-checking with the tin anyway just in case Hawke are not quite up to date.

The sight height is the final piece of the jigsaw puzzle. This is not the distance between the centre of the barrel and the centre of the objective lens! Sight height is the distance between the centre of the barrel and the sightline when measured at the muzzle of the rifle. The easiest way to measure this is to shoot a pellet at some paper at very short range. Mark a cross on the paper, then aim at the crosshairs and shoot. When I say close distance I mean inches, not yards! Unfortunately, there is a problem. You can’t focus on the cross through the riflescope. However, there is a trick. Cover the objective lens with some tin foil and place a pinprick at the centre of the foil. This should help you to see the cross more clearly. Now take your shot. The distance between the cross and the pellet hole on the paper will give you your sight line-height.

plugging the data in

1. Select the Calibre

Once you have installed the application and run it, select the calibre tab for your rifle. Along the top of the application, you will see four tabs: .177, .20, .22 and .25. My rifle is .22 so I will select that one. You can select more than one so that you can compare one calibre with the other on the same graph.

2. Select the Pellet

At the menu bar at the top of the screen, click on “Projectiles” and then select your calibre. A pop-up window will open with a list of common pellet brands and types to choose from. Select your pellet from the list. If the pellet weight is incorrect, you can adjust it by double-clicking on the weight and typing in the correct value.

If your pellet isn’t on the list, you can add it by clicking on the “Add” button and filling out the details including the Ballistic Coefficient.

3. Set the Ballistic Coefficient

The program will fill out the ballistic coefficient (BC) automatically for you in the box next to the weight. If you have obtained your own ballistic coefficient value, enter it in the “BC” box if you haven’t already added your pellet to the pellet database.

If you haven’t got the ballistic coefficient value for your pellet, there’s simply no point in making it up as it is just as important as all the others. With an incorrect ballistic coefficient, you’re just wasting your time.

4. Set the Muzzle Velocity or Power

Enter the velocity of your rifle and pellet combination in the “Muzzle Velocity” box or, enter the power in the “Kinetic Energy” box instead. Only one or the other needs to be set. The application will automatically calculate one from the other.

You need to be as accurate as possible for this parameter so ensure you have measured it with the pellets that you have chosen to shoot with. There’s no point guessing the power or measuring it with a different pellet as that will just give you incorrect results.

5. Set the Sight Height

Enter the sight height in the “Sight Height” box. Try to be as accurate as possible using the method I described above.

This parameter can affect the results quite a lot and is often one of the reasons why at the range people think that Chairgun doesn’t perform as expected. If you input correctly measured information, the results at the range will be spot on.

Be sure to take your time and get the measurements correct!

time to play

6. Kill-Zone

There is a box called “kill-zone” which is set to 1 inch by default. This draws two lines on either side of the centre point (the cross) of your reticle. It is essentially the zone in which you can hit a 1-inch target if you only aim using the centre point of your scope. This area is shaded in pink on the .22 graph and the region where the green arc (the path of the projectile) intersects with this pink box is called the “point-blank range” (PBR).

If you only ever wish to shoot using the centre point, then you will want to make the PBR as wide or long as possible. The quickest way to achieve this is to right-click on the “Zero Range” box and select “Optimum Zero Range”. One of the advantages of using the PBR is that it is not affected by the magnification setting of the scope.

The PBR range is indicated on the bottom of the graph in a white box. It shows the minimum and maximum PBR range, e.g. 6.3 to 31.4 yards with this particular the .22 rifle and scope zeroed at 27.8 yards. If you only intend to shoot out to 30 yards, this might be all you need to do. However, do remember that at the maximum height of that arc, the pellet will be at its highest, i.e. 1/2 an inch above where you aimed. On the graph shown, that’s at around 20 yards. So if you aim at a rat’s head, you might miss altogether unless you aim a little lower. For the same setup, using a .177 pellet, the PBR would be 8.4 to 40.7 yards. You can see why .177 is so popular!

The graph below shows the results for the optimal zero range for my particular .22 setup. I have turned off the reticle intercept lines to make the graph clearer by selecting “Embellishments > Righthand Side Markers > Units” from the top menu bar.

If you prefer to use meters rather than yards then you can make this change at the bottom left-hand corner of the application where the “Range Unit” is shown.

7. Reticle View

If you want to shoot further than the PBR you will need to use what is called “holdover”. This is where you raise your aim (line of departure) in order to hit your target at further distances. To do this, you can use the mil-dots or the half mil-dot markers as your aim points for different distances. Raising the aim or line of departure lifts the trajectory of the pellet and thus moves its impact point further down the range.

If you wish, you could go to the range, spend hours shooting hundreds of pellets and learn which mil-dots to use for various distances. Or you could let Chairgun Pro show you what they are and then confirm them with far less range time and guesswork.

The first thing to do is to select your reticle from the list stored in Chairgun Pro. If you have a Hawke scope, you can click on “Hawke Scopes” from the menu bar and select your model from the list. But if it isn’t listed, or if you use another manufacturer’s scope, click on the “Reticles” menu item instead.

I do not use Hawke scopes. I have in the past but nowadays I mainly use MTC Vipers. These scopes have a mil-dot based reticle that use lines rather than mil-dots. So in my case, I will select the 1/2 Mil-Dot (non IR) or 10x 1/2 Mil-Dot IR option as my scope is a 10x 1/2 mil-dot based reticle. The 10x simply means that at 10 times magnification, the distance between each mil-dot is guaranteed to be known (see your manual for the exact figure). You could use this to calculate the range to the target by knowing the typical size of your target, e.g. an adult rabbit’s head is approximately 4 inches wide from its nose to the back of the head. Then, by how many mil-dots your target covers, you can estimate the range to the target. But to be honest, you soon learn distances by instinct or you could always use a laser range finder.

Chairgun Pro can display a visual representation of the reticle and the distance that each mil-dot and half mil-dot represents. Along the bottom of the application is a row of buttons. Click on the “Intercept” button to show the distance points of your chosen reticle.

You can see from the “intercept view” that the full mil-dots are marked at 28, 35, 42, 48, 54 and 59 yards on the right-hand side with lower ranges on the left-hand side. You might decide that this is good enough and leave it at that. Or, if like me you prefer each full mil-dot to represent 5 yards for the ease of remembrance, there are a few more tricks that I can teach you.

calibration

8. Magnification

So far, I’ve not really talked about the magnification or zoom setting of the scope. I’ve assumed it to be set to the default calibration of the mil-dot reticle, which in this particular case has been 10x. I mentioned first and second focal plane scopes earlier and that for second focal plane scopes, the mil-dots on the reticle do not move as the image is magnified using the zoom control. The benefit of this is that the distance that each mil-dot represents can be changed by adjusting the magnification of the scope.

This is where Chairgun Pro really becomes useful as you can play with the magnification value to your heart’s content without firing a single pellet. This allows you to instantly see the effect that magnification has on the reticle range marks. Increasing the magnification has the effect of reducing the aim point distance between each mil-dot. Whereas decreasing the magnification has the effect of increasing the aim point distance between each mil-dot.

To set the magnification, change the value in the “Ind. Mag” box which means Indicated Magnification or the number shown on your scope’s zoom ring when aligned with the zoom ring marker.

Simply changing the magnification in this example from 10.0 to 12.0 gives aim points of 28, 34, 40, 45, 50, and 54 yards without affecting the PBR of 6.3 to 31 yards. That’s increased the usability of this rifle and scope combination to give a working range of approximately 6 to 55 yards purely using each mil-dot as an aim point with a magnification of 12x.

Do bear in mind that the values on your zoom ring are an indication and may not be “calibrated”. Once you have determined the magnification that you wish to use to achieve the mil-dot spacing that suits you, it’s important to verify that the rifle and scope combination performs as expected in practice. This means range work with targets set out accordingly. Check each aim point and if necessary, adjust the zoom slightly to correct any error.

9. Extending the Range

Should you wish to extend the range of the rifle further, you will need to change the “Zero Range” of the scope to push the aim point distances further out. As always, there are pros and cons involved. The pros, which I have just mentioned, is to extend your shooting range. The cons are that you break the PBR into two smaller zones.

For example, increasing the zero range to 35 yards and adjusting the magnification to 12.5x increases the mil-dot aim points out to 60 yards with 5-yard increments. But it also splits the PBR into two ranges of 5.2 to 11.3 yards and 31.9 to 37.6 yards which may be far less useful depending on your typical shooting distance.

If you want the extra range, then this will be for you. But if you want an unbroken maximised PBR that still has a good overall range for most applications, it’s probably best not to change the Zero Range from that recommended by the application.

range time

All that’s left to do is test your setup at the range. You can either put paper targets out at each mil-dot distance and check how accurate the results are or you could use silhouette knock-down kill-zone targets. If you hit the kill zone reliably, then you know all is well and you can be confident in the adjustments that you have made.

Of course the first thing you must do is zero your rifle and scope at the zero range you have determined using the application. Then test each mil-dot against targets set out at the appropriate distances. Make certain you place the targets out accurately using a sports tape measure or a laser range finder. Pacing the distance out is just wasting all the good work you’ve just put into using the application.

Don’t forget, if you’re not quite hitting the target, adjust the magnification slightly and try again.

wrap up

In this article, I hope you’ve learnt about “point-blank range” and how your zero distance affects it. You’ve also learnt the relationship between magnification and mil-dots for second focal plane scopes and how it can be used to adjust the distance of each aim point.

But more importantly, something you may not have realised, is that this guide also helps you to understand that a high magnification scope is not always necessary. Don’t get me wrong though. High magnification scopes do have their place. For example, field target shooters use them to accurately focus on their target. This in turn provides a distance measurement to the target from a large focussing sidewheel. This might be good for field target shooters, but for general shooting, what a faff!

I hope you’ve found this guide useful and informative. I should add that there are many more useful features in Chairgun Pro just waiting for you to find and use them!