Bending airgun barrels: Part 1

by B.B. Pelletier

A couple weeks back, we talked about straightening bent airgun barrels to improve accuracy. We want to do that so we can hit targets with the sights that were installed. There is, however, another reason for bending barrels. Some guns have sights that do not coincide with where their barrels are pointing, even when they’re not bent. For this situation, we also need a fix.

Many guns have replacement rear sights. I own a BSF S70 breakbarrel rifle that has a Williams peep sight that was either installed by Air Rifle Headquarters (the original company in Grantsville, West Virginia) or was a sight they sold for owners to mount. ARH did inform the customers of the necessity for the rear sight to have a complimentary taller front sight installed, and on my rifle that didn’t happen. I think this was an owner-installed sight that they probably hated ever since.

This is the problem I’m faced with.

The special Williams peep sight is low and fits the rifle well. It looks good, and I want to keep it.

I like the vintage look of the original front sight. Bending the barrel is the only solution!

In either case, the rear sight cannot be adjusted low enough for the rifle to strike the target at 10 meters. Since 10 meters is such a common shooting distance for an air rifle, this is not handy. The other possibility would be to raise the front sight higher, but I don’t want to do that. I happen to like the look of the front sight that’s there and want to leave it as installed. My only option is to bend the barrel.

Several blog readers, including Kevin Lentz, commented on having bent many airgun barrels and how easy it is to do. My buddy Mac has also bent a number of airgun barrels to get them on target.

While a barrel may be bent in any direction, up is by far the most common direction you’ll have to go since the majority of breakbarrel rifles shoot a little low. The second most common direction is down, which is what I need to do to fix the kind of problem I have.

I was in my reading room a few days after that; and from the pile of literature lying on the sink, I picked up the 2000 Edition of The Gun Digest and stumbled across an article by Todd G. Lofgren titled, Sighting In Single-Actions. The author describes, shows and tests the results of bending the barrels of numerous Colt Single Action Army revolvers to get them to shoot to the point of aim at 25 yards. He knew that the traditional way of doing this is to either file down or add to the front sight for height and to bend it (the front sight blade) in the direction opposite of where he wanted the bullet to go, but that didn’t appeal to him. He built a jig and used a 12-1/2 ton hydraulic press to actually bend the barrel in the direction the bullet needed to go.

He fixed guns that were off in all ways, but by far the most common directions were to the left and low. And then he shot three groups at 25 yards to prove the guns now shot to their point of aim. Before bending each barrel, the extractor housing was removed; and in every case, it was installed after the bend without a problem, thus proving that the bend itself was only a very small distance.

Lofgren commented that the first-generation Colt barrels are easier to bend than the barrels of guns made today. That means their metal is softer and more ductile, and lends itself to slight deformation better than barrels made from harder steel. That bodes well for airguns, because they’re also still being made of soft steel that should deform easily.

Lofgren also happened to favor the short 4-3/4 inch barrels, and all of the guns shown in his pictures have barrels of that length. Compared to that, bending a 12-inch or longer air rifle barrel made from thinner steel stock should be a piece of cake!

While he uses a hydraulic press to bend his barrels, I think that bending an air rifle barrel that’s sitting between two blocks 12 inches apart will be easy enough to do with a common screw like the kind found on a C-clamp. If the jig is constructed correctly, it should be possible to control the amount of pressure very precisely, which is desirable for collectors who don’t want to ruin their fine guns.

What about guns with fixed barrels?
It should be possible to bend guns that have fixed barrels, as well, provided the barrels are solid. This process will not work on barrels inside jackets or shrouds, which lets out many airguns of modern design.

Don’t over-think this!
Some readers might think this operation through and wonder if bending the barrel in the direction you want the pellet to move is correct. If you bend the barrel, you also move the front sight — and we know that the front sight is supposed to be moved in the opposite direction that you want to pellet to move. But Lofgren cautions his readers not to over-think this and just bend the barrel as they want the strike of the round to move. It’ll work out perfectly that way.

This fixes bent barrels, too
The initial reason for bending barrels was to straighten them after they’re bent from an accident or from their manufacture — not because they weren’t hitting where the sights are aiming. But one bend is the same as the other. It’ll work for both problems — I guess. At any rate, seeing a man bending the barrels of collectible first-generation Colt revolvers and getting the results he was after has given me the courage to try the same thing on this air rifle.

The next step is to damage a spring-piston barrel and then try to bend it straight again. If I can do that, then bending the S70 barrel shouldn’t prove too difficult. In the process, I hope to construct a simple low-cost barrel-bending fixture that will serve all my future needs. It should be a fun experiment!

Can a fixed-barrel airgun have barrel droop?

by B.B. Pelletier

This report is in response to a comment Pyramyd Air got from a customer who doubts that fixed-barrel airguns can ever droop. His position is that they can only have droop if the barrel is heated in some way (as on a firearm that fires very fast) or if the gun is assembled in a shoddy fashion.

He said he believed barrel droop is only commonly found on breakbarrel airguns, which is why he said he would never own one. He thought that droop was mostly caused by the metallurgy of the barrel.

Today, I’d like to address the subject of barrel droop in detail. It can be caused by many things, but poor metallurgy isn’t one of them. Barrels do not bend from cocking, despite what some people may think. It is true that a barrel can be bent by human force, but the force required to do so is much greater than the heaviest cocking effort on the most powerful magnum airgun. So, poor metallurgy is not a contributor to barrel droop.

What is barrel droop?
I will explain what barrel droop is in detail later in this report. For now, I’ll just say that barrel droop is a condition in which an air rifle shoots so low that the scope cannot be adjusted to hit the target.

You must understand that most scopes cannot be adjusted all the way to their highest elevation settings and still operate correctly. This will differ from scope to scope, but generally most scopes do not work well when adjusted above three-fourths of their maximum elevation. It’s imperative that they get on target before reaching that height, and a drooping barrel can prevent that.

Throughout the first five decades of spring-piston air rifles, no one ever heard of barrel droop. It was a non-issue. That was because nobody bothered scoping their air rifles.

The sights on most breakbarrel guns are attached to the barrel, both at the front and rear, so they’re in line with the bore — as long as the bore is drilled straight through the barrel, which it seldom is. The amount of misalignment is usually measured in the thousandths of an inch — an amount the sights can easily account for.

With both the front and rear sight attached to the barrel, there’s less chance for misalignment.

In the 1960s, retailers began attaching scopes to airguns to sell more of them. Firearms had been using scopes for some time, and the general belief among shooters was that scopes extracted the maximum accuracy from any gun.

But scopes had a problem, as well. They were attached to the spring tube of the gun, which isn’t integral with the barrel on a breakbarrel airgun. For the first time, the alignment of the spring tube and barrel came into question.

It soon became known that most breakbarrel guns have a barrel that slants downward from the axis of the spring tube. In the 1960s and ’70s, breakbarrels were hand-selected for scope use when they exhibited less slant than other guns of the same model. You can read about this selection program in both the Air Rifle Headquarters and Beeman catalogs of the period.

What those catalogs didn’t address was the fact that fixed-barrel airguns can and do sometimes have the same barrel slanting problems. They didn’t address it because, at the time, scoping airguns was brand new and not that much was known about it. The people scoping the guns often installed simple fixes, such as shimming the rear ring, and didn’t even think about why they were doing it.

Why the barrel droops
The comment that prompted this blog went on to say that barrel droop was caused by poor metallurgy. Evidently, the writer thought that “droop” referred to a barrel that was curved (or bent) downward — which is not the case. The term “droop” doesn’t refer to a barrel that is somehow curved. It means a barrel that points in a direction away from the sight line, so the axis of the bore and the sight line are diverging. To correct for this droop, the scope has to be repositioned to align with the axis of the bore.

We all understand that a pellet starts falling the moment it leaves the muzzle. The farther from the muzzle it goes, the faster it falls; so the line of flight is actually an arc, rather than a straight line. To align the sight line of the scope with the axis of the bore, we have to align the scope to look downward through the line of flight. To be effective — that is to get any distance over which the pellet is on target — the sight line is made to pass through the arc of the pellet twice — once when the pellet is close to the gun and again when it’s farther away.

The scope is angled down through the pellet’s trajectory. This illustration is greatly enhanced for clarity. This alignment is done the same for firearms and airguns, alike.

But the question is, “Why does the barrel point downward?” With a breakbarrel, it’s usually because of how the breech locks up at a slight angle that causes the downward slant. Some guns, most notably target breakbarrels, overcome this with barrel locks that cam the breech tightly against the spring tube in a straight line. Most guns rely on the spring-loaded detent to both align and hold the barrel during firing. If there’s a weakness, it’s at this point. When a breakbarrel with an unlocked breech fires, the barrel tends to flex in the direction the barrel is hinged. If the barrel broke upward to cock, the problem would be reversed and we would have a barrel “climb” problem.

A breech lock like the one on this HW 55 ensures that the barrel always aligns with the sights — provided the rifle is designed that way.

Do you now understand that the barrels are perfectly straight, and it’s just the angle of the bore’s axis relative to the line of sight that creates the drooping problem? Good, because that’ll make the following easier to understand.

What about underlevers and sidelevers with fixed barrels?
How can a fixed-barrel rifle have droop? Easy — the barrel isn’t attached to the gun with the bore parallel to the line of sight. Presto! Automatic sighting problem. Or the scope base that’s attached to the spring tube may not be aligned with the axis of the bore. Or the bore may be drilled off-center; and although the outside of the barrel is parallel to the sight line, the bore’s axis isn’t. Any of these three things can happen.

Bore not drilled straight
This is very common. It’s extremely difficult to drill a deep (long) hole straight through a steel bar. The drill bit can wander off-axis as it bites its way through the steel, or it can be off-axis all the way through the bore if it isn’t correctly set into the holding fixture before the drilling begins. I’ve had barrels with bores as much as a quarter-inch off-axis with the outside. Granted that’s extreme and uncommon, but it demonstrates the possibility.

The only way a barrel-maker can ensure concentricity of the bore to the outside of the barrel is to machine the outside of the barrel after the gun is rifled.

Barrel isn’t aligned with the spring tube
This problem is also common. When the barrel is pressed into the spring tube (usually into a block that’s held in the front of the spring tube), the bore isn’t aligned with the spring tube. You might think that modern manufacturing processes make perfect things time after time, but the truth is that there’s always some variation.

Scope base on top of the spring tube is not aligned with the bore
Of all the problems with scope alignment, this one is the most common. Off-axis bores are usually held to just a few fractions of an inch for which the scope adjustments can easily compensate. The same is true for barrels that are bushed off-axis. But scope bases are both short as well as attached in such a way (by spot-welds and rivets) that precision is difficult to maintain. Because scope bases are short, any small deviation in their positioning is exaggerated when extended out to infinity by a scope’s sight line. This is the one place where firearms and certain brands of airguns have an advantage over other brands, because they machine their scope bases into the receiver (of a firearm) or scope tube, rather than riveting or spot-welding the base to the scope tube. If the tooling is set correctly, the machining process ensures alignment of the scope base.

Talking about the spot-welded and riveted scope bases brings us to a discussion of one well-known company that makes highly regarded spring-piston air rifles. This company stands head and shoulders above the others when it comes to having barrel droop — both with their breakbarrels and their fixed-barrel air rifles. That company is Diana. Historically, enough Diana air rifles have had barrel droop so severe that special corrective scope mounts have been made and successfully marketed for their models. Even RWS, who exports Diana airguns, has marketed such a corrective scope mount.

But even Diana can change. Their most recent breakbarrel is their 350 magnum model in all of its various forms, and this rifle is very noticeably immune to the drooping problem. Something has changed at Diana. I would think that, over time, we’ll see this change spread to all of their models.

Firearms also have droop
Drooping isn’t just an airgun problem. Firearms have droop, too. But because of how firearms were scoped in the early days, nobody noticed the problem.

When firearms were scoped back in the 1940s and ’50s, many of them did not have optional scope mounts available. It was very common back then for a gunsmith to drill-and-tap holes into the firearm to accept scope base screws. Naturally, when a gunsmith did the job, he would align the holes in the scope mounts so the axis of the barrel was in line with the sight line seen through the scope. If there was any barrel droop, it was corrected as the mounts were installed.

Do barrels only droop (slant down)?
Before someone asks the obvious question, I’ll address it. Yes, there are airguns with barrels that slant up, plus point to the left and to the right too much for the scope to compensate. They’re not encountered as often as droopers, but they’re not unheard of. The reasons for most of these problems are the same as for droopers except for one standout reason.

If a breakbarrel rifle has been fired with the barrel open, so the barrel was allowed to snap closed from the force of the mainspring, that rifle will have a bent barrel. The barrel will be bent upward at the point it emerges from the baseblock, which is the piece that holds the barrel in the action. It’s where the pivot bolt attaches. It’s the blocky-looking piece the barrel is coming out of in both photos of guns in this report.

For this type of problem, the solution is to bend the barrel straight again. Any qualified airgunsmith should be able to straighten a barrel that has this problem, and a number of owners have learned to straighten their own bent barrels..

Most airgun barrels don’t droop
To put this report into the proper perspective, I should mention that a drooping barrel isn’t that common. I have several air rifles whose barrels are okay for shooting with scopes as they came from the factory. And, of the hundreds of rifles I test, only a small percent have a drooping problem. So, it isn’t a given that your rifle will droop.

But you may get a drooper, and you can rest assured that there are plenty of solutions to rectify the situation should you encounter it. The things to remember are:

Not all breakbarrels droop. Only a small percentage do these days.

Rifles with fixed barrels can also have droop, for the reasons mentioned in this report. It is not as common to find a fixed barrel with droop, but any air rifle that has a separate scope base that’s either spot-welded or riveted in place is a likely candidate for droop.

Firearms have droop, just like airguns. But the amount of droop is small enough that it’s corrected by the scope or by the mounts that are supplied by the firearms manufacturers.