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.
History
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 read more
