Leading-in a barrel. What is it and why you need to do it…

I’m sure many will have heard the term leading-in a barrel. Or perhaps you’ve heard of it without realising by being told to run in a new air rifle by shooting a tin of pellets through it. I have heard this many times over but never seen any documented evidence of this. So I thought I’d have a go at recording the effect in practice.

The theory is that barrels have imperfections. As you shoot pellets through the barrel, each leaves a deposit in the imperfections which gradually builds up forming an evenly bored barrel. The result should be evident with smaller and smaller groups.

I started by first cleaning out the barrel on my Theoben Rapid MKII with a Napier pull through kit. This was to ensure it was clean and unleaded. I had previously performed a group test on this rifle so I knew which pellet it preferred.

I always do group tests at 50 yards. I find this provides the best way to determine a real difference in pellet groups. At shorter ranges, often more than one brand of pellet may produce similarly sized groups. This is ok if you only ever intend to shoot at that range. But should you consider shooting at further distances, you will not really know how your chosen pellet will perform and whether it will reliably hit the target or kill zone. If you hunt with your air rifle, not knowing how your pellets group with certainty is, quite frankly, irresponsible.

Once at the range I placed a sheet of A4 on a sheet of box cardboard down the range at 50 yards. On the sheet, I drew twelve crosses as targets to aim at. The purpose is not necessarily to be on target but to consistently aim at the same point shot after shot. Where the pellet lands is irrelevant when checking how well the rifle and pellet performs. The primary purpose is to see how tight the pellets group.

Back at the firing line, I use a deadshot shooting bag filled with dry uncooked rice. Whilst these are indeed heavy, they provide an ideal malleable and firm support for your PCP air rifle. I wouldn’t use one of these for a spring air rifle though. For those, I would use a bag filled with polystyrene balls which can also double as a cushion to sit on in the field.

three shot groups are not sufficient!

Firing just three shots really isn’t sufficient. I find ten shots to be the optimum number to determine group size. It’s very possible to have three good shots and others spread further afield. Stopping at three is potentially misleading yourself.

With the magazine loaded with Air Arms Diabolo Field pellets, I fired the first ten shots. I could already see that the group was huge! I turned the magazine around and fired the next ten shots at the same target point. I repeated this two more times of 20 shots. Each had similar huge groups.

I then switched to ten shot groups as I was expecting the groups to improve. But this was not the case for rounds 61-70. Then something changed. The next ten shots (71-80) were aligned horizontally with little vertical variation. I knew this was caused by the wind that had picked up and that without the effect of the breeze, it would probably be grouping quite well.

I decided to move to a sheltered part of the range. This lane has been specifically constructed with a fence running the whole 60 yards on each side of the lane with each yard numbered. Once I had set up the target and rifle at the bench, I took a breather to calm my heart.

The next ten shots (81-90) was better, but still not as good as I know the rifle is capable of achieving. But the following group (91-100) really began to tighten up. Apart from two strays, it was down to 3/4 of an inch. It wasn’t until two further test groups that I knew the rifle had achieved its full potential or at least as good as I could probably shoot!

This final group, shots 111 to 120 were overlapping each other at a group size of 1/2 an inch or less than a five pence piece!

So there you have it. The effect of cleaning your barrel on your groups and how leading in the barrel can improve the group size. This may not necessarily be true for all barrels. Some may prefer a clean barrel. Some may take fewer pellets to lead up, some may take more. You won’t really know without testing your rifle and pellet. It’s also quite possible that switching pellet type and/or brand could strip the barrel of any lead that has already built up causing poor groups.

Something else to watch out for is lead fouling. This is where too much lead may result in poor grouping. If your groups begin to open up, it might be time to clean the barrel and start over. That may not be the only reason. It is possible that the pellet manufacturer has altered the pellet somehow. Or your barrel or scope may have worked loose. Or maybe you are having a bad day.

Oh, almost forgot. I shoot pellets straight from the tin. No cleaning and no lubrication.