Bending airgun barrels: Part 5
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, I’ll apply what I learned about bending an airgun barrel to a real problem. As I said in several earlier posts, I have a BSF S70 breakbarrel that came to me with a peep sight installed but no open sights. BSF rifles aren’t common in the U.S., so finding a correct rear sight would take some time; but more importantly, I like the peep sight that’s on the gun. It was one that Air Rifle Headquarters sold as an optional sight, but the owner of this rifle removed the original sights and didn’t replace them when he sold the gun. So, all I have is the peep.
When I tried to sight in the rifle, I discovered that it was shooting 2-3/4-inches high at 10 meters, which would put it even higher at 25 yards. Either the front sight had to be raised (because the peep was adjusted as low as it will go), or the barrel had to be bent. Since the front sight is dovetailed into the barrel, I decided to bend the barrel. That was two years ago. Since then I’ve been researching ways to bend barrels and thinking about the equipment needed to do the job.
This report has already documented the simple fixture I constructed, and you can read how it worked in the earlier parts. Now, I’m going to bend the barrel of this pristine collectible air rifle so I can shoot it and hit what I’m aiming at.
The barrel-bending fixture is quite simple to set up. It uses a c-clamp to apply steady pressure on the barrel and a ratchet wrench turns the clamp slowly, so the bend is under complete control.
This was the final bend. This barrel is softer than the first one, but it springs back when the tension is released.
The S70 didn’t fit the fixture the same as the first rifle did. I had to make some adjustments, but there’s enough flexibility built in to allow that. As I started the bend, I noticed right away that this barrel was bending much easier than the other one had.
Remember what I said about metallurgy of the various airgun barrels? I said that you should treat each new barrel as though you were bending a barrel for the first time. That was good advice for this job, because the S70 required a lot less pressure to bend. So I went slow.
The beauty of this fixture is that you can take the rifle out and shoot it after each bend. So that’s what I did. Let me show you the first target, which explains how the work went. I shot the same JSB Exact 8.4-grain pellets as were used when the gun was first fired.
This target documents all the work that was done. It is explained in the text below. The group below the bull is the final 5 shots, 3 of which are in one hole under the number 5. The one at the lower left was fired before I was ready but didn’t bother me.
I knew the rifle was shooting high, and I had the former target that told me how high. So, I first bent the barrel a little then shot the rifle to see what had happened. The first shot after the first bend was still high, but the point of impact had dropped over a quarter-inch. That told me the S70 barrel was bending very easy.
I bent the barrel a second time and put another pellet in the same hole as the first. Then, I bent it once again (three bends so far), and the third shot was the same height but to the right of the first two. Obviously, I had to be more aggressive.
The fourth bend was more aggressive, and the shot that followed dropped into the black bullseye for the first time. Next, I bent the barrel more aggressively, again, and the shot dropped to almost the middle of the red center.
Bend six was even a little more aggressive, and the shot dropped a little more and went to the right. Bend seven was about the same as six, and that shot went into the same hole as shot six had.
Now I knew the barrel needed a lot more pressure to go as far as I wanted. So, bend eight was the most aggressive of all. It’s the one pictured in the photo above. The shot that followed was below the aim point for the first time, which was what I was after. Four more shots were in the same area, though one of them went off before I was ready and struck the target low and to the left. From this result, I knew I was done bending the barrel.
It was time to sight in the rifle with the adjustments on the Williams peep sight that was on the gun. This was the first time since that rifle first got the peep sight back in the 1970s that it’s been able to shoot to the point of aim at 10 meters!
The sight-in took a long time because the peep sight adjusts in very fine increments. But I managed to walk the pellets up the target until the final one was at the correct height. Now it was time to verify that the gun still shot well. I have the original target for comparison.
As you can see, the rifle shoots as well as before. All that’s been done is adjust the point of impact so the sights can be used at close range.
The Williams sight has a number of screws that are used to lock the sight in position, once it has been sighted in. They all have to be loose to adjust the sight; and when you tighten them at the end, they can make the POI move just a little. You have to check your work after you think the sight is locked down.
So, the fixture I made works for real-world applications, too. It doesn’t take up much space, and I’ll keep it around for any jobs that might arise in the future.
Bending an airgun barrel isn’t something to do lightly; but if you have to do it, it’s nice to know the job can be done with a minimum of tools and time. Today’s project took a total of 45 minutes, which includes bending, shooting and sighting-in the gun at the end of the job.