Bending airgun barrels: Part 4
by Tom Gaylord, a.k.a. B.B. Pelletier
Jedediah Strong Smith submitted this week’s winning photo for BSOTW.
Do you see that there are two people in this image?
As promised, today is a continuation of the barrel-bending exercise I’m doing with the fixture we looked at yesterday. When we left things, I’d bent the handle on the c-clamp I was using and couldn’t get the barrel to bend enough to shoot to the point of aim at 10 meters. After that, I used the cross-members of my workbench for leverage and bent the barrel some more. I did get it closer to the aim point, but not close enough. And the scope was still adjusted as low as it will go, which is not what I wanted.
Today, I’ll tackle the job with a slightly different tool. Remember that the goal is to straighten (or bend) a barrel enough that the gun shoots to the point of aim at 10 meters.
I needed some kind of pressure-generator that didn’t get ruined when I put some serious torque on it. The screw on the 6-inch c-clamp I used is strong enough for the job, but the soft steel T-handle can’t take the strain that had to be put on it. What’s needed is a c-clamp that isn’t prone to bending.
I talked it over with a shooting buddy who told me he might have the perfect tool for the job. It was a 6-inch c-clamp that has a hex nut on the end of the screw. Instead of a sliding T-handle, you tighten this one with a socket held in a ratchet wrench. But it was also a different type of c-clamp, altogether. It was obviously designed for pressing out a large pin of some kind. That didn’t bother me, and the fact that the screw accepted a socket wrench seemed like exactly what I wanted. I envisioned adding pressure in small amounts with the ratchet wrench, so the control I was after would be even greater than with the first c-clamp. But there was also a problem.
The end of the c-clamp screw came to a sharp point — what was needed to press out large pins, but not good for pressing against wood — even oak. That would have to be changed.
My buddy said he had bought the clamp for two dollars in a pawn shop years ago and had never found a job for it. This sounded like the perfect application to him, so he donated it to the cause. He also made a flat cylinder to cover the sharp point on the end of the screw so it wouldn’t bite into the oak post too much.
He ran it by my house a few days later, and I resumed the barrel bending (straightening) exercise. You will recall that when we last looked at the gun, the barrel was shooting about 1″ to 1.25″ high at 10 meters with the scope adjusted as low as it would go. I wanted it to shoot below the aim point at 10 meters so I could crank a little elevation back into the scope. That was just so I could go both ways with the adjustment knobs — not that I really needed to sight the scope for anything less than 10 meters.
I put the rifle back into the fixture, put the new clamp in place and began applying pressure. Immediately I saw that this was going to work much better than the other c-clamp, because I could get a lot more torque on the ratchet handle than I could on the c-clamp T-handle with my makeshift cheater bar. So the first time I used it was the only time I needed to use it — it finished the job in a single try!
The new c-clamp worked better than the first one. I was able to bend the barrel exactly like I wanted the first time! The bend seen here sprang back when the tension was released. That’s a piece of leather between the clamp and the barrel.
I knew the barrel would spring back after it was released from the fixture, so I bent it a little farther than seemed necessary. Sure enough, when the pressure was released, it did spring back. I could see a slight downward bend in the barrel for the first time since it was slammed shut, so it was time to shoot the rifle again.
The next picture is complex because a lot is happening. I’ll explain it in the text, rather than the caption, so it’s easier to follow.
This is the first target after using the new barrel-bending clamp. Read the text below for an explanation.
The aim point for all shots on this target is the center of the top bull. The first shot landed about 3.125″ below the point of aim. This was the first time in this test that the pellet landed below the aim point, and it meant the bend had finally been successful. I adjusted the scope up several clicks and shot again.
You can see that shots 2 and 3 are progressively higher on the target but are still below the aim point. The 4th shot, however, is considerably higher than the aim point. Now it was time to adjust the elevation back down and zero the scope.
The first down adjustment wasn’t far enough, but the second one was right on target. But look at the large distribution of shots. Had I destroyed the barrel’s accuracy by bending it? I didn’t think so, but I needed to shoot some more, to know what was happening.
The shots seem too scattered. The rifle was capable of better groups before the bend, and during it, too!
The next target just confirmed what I already knew. The shots were on the aim point, but the group was too large. What was wrong? I wondered if the scope was now adjusted too high and the erector tube was floating. Did I need to bend the barrel back in the opposite direction? I didn’t think so, but this open group was puzzling.
The way to determine whether the scope adjustment is too high is to adjust it lower and shoot another group. I did this, and it seemed to shoot a little tighter but was hard to tell for certain.
The scope was adjusted down for the first group on the left. Then the broken shell of the muzzlebrake was removed for the second group. The first three shots of the second group went into what looks like two pellet holes at the top of the group.
Then, I remembered the lengthy Pellet velocity versus accuracy test I did late last year, in which it was demonstrated that vibrations affect accuracy more than velocity. What I haven’t told you before now is that while I was bending the barrel by hand in the crotch of my workbench, the plastic front sight assembly broke and half of the sight fell off the gun. The hollow shell of the assembly was still attached, loosely, to the barrel. Could that be the cause of the scattered shots?
The lower piece of the front sight assembly fell off the gun after I bent the barrel using my workbench. That left the large hollow shell (above) still attached to the muzzle. I removed this piece, thinking it was creating accuracy-destroying vibrations.
I removed what was left of the front sight assembly, except for an anchor that seems to be epoxied on the barrel. It shouldn’t cause a problem because it’s very tight. Of course, it does change the vibration nodes just by adding a little extra weight, but I left it on the barrel because I didn’t want to fool with it.
Then, I shot a last group. The first three pellets went into a very tight hole and the last two went into a second smaller group to the right of the first. It isn’t the best group I’ve shot with the gun, but I’m satisfied the gun is shooting as it should. I believe it was the vibrations caused by the remaining hollow plastic muzzlebrake shell and not the elevation of the scope that caused the scattering of the shots.
At this point, I was finished with this test. The fixture had done everything I wanted it to, and I was satisfied that this approach will work. I don’t recommend doing this unless you have a real problem with a gun, because there’s always a lot that can go wrong. But I think this procedure is precise enough for this kind of repair.
Some thoughts on bending airgun barrels
First, let’s remember that I have been dealing with a severely bent barrel in this project! You’ll never get a production air rifle with a ski-jump barrel like this one. But you may get a rifle that doesn’t shoot to the aim point for a number of reasons — wrong sights, bore not drilled straight, scope base not aligned with the bore or perhaps somebody bent the barrel before you got the gun. You may never need to bend a barrel as much as I have in this report, and bending it very slightly to correct a couple inches of impact shift will be much easier than what you’ve seen here. But this technique works well and gives you wonderful control over the process.
Can you bend the barrel in different places? Of course! All it takes is positioning the bending blocks and the pressure point where you want the bend to be. Just remember that bending an airgun barrel precisely isn’t easy. Anyone can bend a barrel, but to get it the way you want it takes control over the process.
Next, I think there must be differences in the metallurgy of various airgun barrels. None of them are ever hardened purposely, but different steel alloys will respond differently to pressure, I’m sure. Different rifling techniques, such as button rifling and hammer forging, undoubtedly create different amounts of inadvertent work-hardening. You should approach any barrel-bending project as though it’s the very first time you’ve done it — because when you consider the metallurgy issue, it just might be!
I also think that once a barrel’s been bent in a certain direction, such as the corrective bending I’m doing here, subsequent bends in the same direction are easier to make. That’s just a guess. If I’m right, it means you have to go slower and be more careful on bends 2 through N.
This report began with my complaint that my BSF S70 doesn’t shoot to the point of aim. It has an aftermarket peep sight that cannot be adjusted low enough to get the point of impact to coincide with the aim point at 10 meters. And I don’t have a factory original rear sight for the rifle. So, I want to bend the barrel down to correct the situation. It won’t take anything close to the amount of bending I’ve had to do with this rifle, so I shouldn’t be able to notice the bend after I finish. That’s important because the S70 is in beautiful original condition.