Choked bores and tapered bores
by B.B. Pelletier
This subject came up as the result of a comment I made about choked and tapered bores. It turns out that gun makers were having this same discussion 140 years ago with pretty much the same results.
The best gun makers of the 1860-1910 timeframe (and Harry Pope for just a little longer) all either taper-bored their barrels or choke-bored them. I will describe each of these conditions in a moment. There really isn’t much difference between choke-boring and taper-boring, but the slight difference that does exist allows us to talk about each of them as a separate issue.
Most gun makers (or barrel-makers, because in many cases — like Pope, a man did not make the entire gun) did taper-bore their barrels. But that wasn’t what they called it, so the fact that they did it got lost because of the subtleties of the language.
What is a tapered bore?
A tapered bore is exactly what the title implies. The diameter of the bore gradually tapers down from breech to muzzle. The amount of the taper is slight — perhaps one-thousandth to as much as two-thousandths of an inch; but at the time this service was performed, the measuring tools needed to accurately measure it weren’t commonly available. So, most of the makers didn’t actually know how much they were tapering their bores — just the fact that they were.
What does this do?
Why taper the bore at all? Because there are advantages — the primary one being an increase in accuracy. The reasons for this increase are less obvious and not entirely understood — or perhaps I should say they’re not entirely agreed upon. We know taper-boring works, but exactly why remains something of a mystery.
One thing that we do know is that when the barrel squeezes the bullet down smaller, it prevents gas blowby, which is damaging to the bullet because it erodes the sides and unbalances it. But in guns that use black powder for the propellant, the hammer blow of the exploding powder actually squashes the base of the bullet outward to make firm contact with the sides of the bore. This is called obturation. A black powder rifle doesn’t need a choked bore to prevent gas blowby, because obturation already addresses it — as long as the bullet is fitted closely enough to the bore to begin with.
So tapering the bore must do something else, because it works for black powder arms just as it works for those guns that use smokeless powder that does not obturate the bullet. The theory that I believe is that a tapered bore grabs the projectile more firmly just before it exits the barrel. It stops any unwanted vibrations and sends the bullet on its way with no instability. It ends any side-to-side play the bullet might have inside the barrel. Just because the base of the bullet has been squashed larger by the force of the exploding gunpowder doesn’t mean that the entire length of the bullet is equally in contact with the bore; but if the bore narrows down enough, there will be no doubt about it.
How they did it
Now that you know what a tapered bore is, let’s find out how the barrel makers managed to do it. Actually, the process is simple. If you read how gun barrels were made back in 1840-1910, you’ll see that they did it as a matter of course. They called it “leading the bore” and by that they did not mean lapping the bore, which is a similar but separate step that some but not all barrel makers did.
When they “leaded the bore,” a bore-cast lead slug that was charged with emory was passed back and forth through the just-rifled bore until it had removed a tiny bit of metal from the inside. To do this, they first inserted a long bore-fitting wooden dowel down the barrel. The front section was turned down much smaller than the bore.
The rod was entered from the breech and positioned with its end flush at the muzzle. The barrel was next heated until it was hot to the touch, then molten lead was poured down the muzzle until it pooled up flush with the muzzle. The lead was stopped from going down the bore by the bore-sized wooden rod that was not turned down, and it attached itself to the smaller diameter portion of the rod near the muzzle. When the barrel cooled down, the rod was pushed out the muzzle and the lead mass that was on the end was removed. It was then trued up at both ends, and the wooden rod was pushed out, leaving about a 1/4-inch hole down the center of the plug. A small groove was cut in the lead cylinder, then the cylinder was screwed onto a tool-steel rod that was called the leading rod. The lead plug was then rolled on a steel plate that had emory powder spread upon it. The leading rod was free to turn in its handle, so the lead plug could follow the pattern of the rifling.
The leading bolt after cleanup looks like this. It’s then screwed onto a tool steel rod that works it inside the bore. Image copied from “The Muzzle-Loading Cap Lock Rifle” by Ned H. Roberts, copyright 1952.
At this point, the inside of the bore received a light coat of fine oil, such as sperm whale oil. The emory-charged rod with its lead slug was then carefully inserted at the breech, making sure to engage the rifling exactly. The rod was then moved back and forth from the breech to within about three inches of the muzzle. By concentrating on the rear of the barrel and only going forward a relatively few times, they controlled the amount of metal that was being removed from each part of the bore.
Occasionally, the rod was partially withdrawn at the breech but never again fully removed. When it was exposed in part at the breech, more emory powder could be applied along with a little more oil. By never completely removing it, the lead slug always remained in the proper engagement with the rifling. When the lead slug wore down, the tool steel rod was screwed into it more, forcing the sides back out and into the bore of the rifle.
The rod was worked back and forth, with more time given to the portion closer to the breech and less as the lead slug approached the final three inches of the barrel. How long this procedure took varied with each maker, and probably with the type of material they were working with — i.e., soft iron, cast steel, compressed steel, etc. Undoubtedly, the exact process was a closely-guarded secret for each maker. But it did work, and what they got was a bore with a gradual taper from breech to a point about three inches from the muzzle. Since they never went past that point, that section of the bore remained a true cylinder and was the tightest point in the barrel. It was the choke point.
Since these are muzzle-loading arms and the muzzle is also the tightest point of the barrel, some of you may be wondering if the bullet wouldn’t squeeze down when it was initially loaded and then lose contact with the bore after passing the choke point. That’s exactly what happened, and it made the rifle much easier to load!
Remember obturation? When the black powder exploded, the bullet was upset by the force and enlarged to grab the bore tightly. TWhen it encountered the choke point, everything happened just as I’ve described above. This gave the bullet remarkable stability that had not been seen previously.
Many riflemen were no longer using patched round balls when this style of rifling came into vogue. They were starting to experiment with conical bullets, first with the sugar-loaf or picket-style, then later with the longer, heavier cylindro-conical shape.
The picket bullet or sugar loaf bullet (left) was an early first replacement for the round ball in rifled guns. It has a very short bearing surface that makes it easier to load, but also makes it susceptible to tipping inside the bore. It more than doubled the accurate range of the rifle but required extreme care when loading. The cylindro-conical bullet on the right has more bearing surface but also needs to be driven much faster to stabilize when fired from a barrel of a given twist.
There’s much more, but not now
Bullet shapes of the late 1800s are a fascinating study. For instance, were you aware that some expert riflemen favored a hollow-based cylindro-conical bullet as the most accurate type? For now, let’s leave the world of firearms and return to airguns because choking has a definite place there, as well.
The choked bore
I haven’t described the difference between a tapered bore and a choked bore, so here we go. A choked bore is really just a tapered bore with a short taper. In other words, the bore is parallel from the breech to the choke, and then in a short distance of less than a half-inch the bore tapers down to a smaller diameter that stays parallel until the muzzle. In firearms, this distance for the choked part of the barrel was about three inches, but in airguns it’s more like two.
Intentional versus random and accidental chokes
The only intentionally choked airgun barrels I know of are made for pneumatic guns. Let’s examine why. The pneumatic is much like the firearm that uses modern gunpowder. Instead of a sudden, violent explosion, smokeless gunpowder burns at a reserved rate of speed. When confined, this rate is extremely fast, but it still cannot be called an explosion. So, modern smokeless gunpowder does not deliver the same hammer blow that obturates bullets. Nor do pneumatic guns blast out pellet skirts into the walls of the bore, which is very similar to obturation in the airgun world.
Pneumatic guns release their air at a restrained rate that, while it sounds sudden to us, is really measured in milliseconds. A lot of air is released when a pneumatic gun fires; and though the pressure in the barrel continues to decline as the pellet moves down the bore, this pressure is still enough to provide continued acceleration all the way to the muzzle.
Because the air pressure is restrained in a pneumatic, the pellet skirt is not enlarged and pushed into the wall of the bore. But in a spring gun, it is. A springer releases just a tiny bit of highly compressed air in an instant. This rapid burst of pressure is enough to swell the skirts of some pellets, making them have better contact with the bore.
So, to better stabilize pellets in pneumatics and remove any variations they might have, a choked bore is ideal. Therefore, all of the finer precharged, single-stroke and multi-pump airguns have choked bores. You can feel this if you push a pellet from the breech to the muzzle with a cleaning rod. The pellet will encounter resistance about two inches from the muzzle.
But spring guns don’t need a choke, since the act of firing swells the pellet skirts. However, some spring guns do have the same resistance near the muzzle that is felt in better pneumatics. This is an accident of swaging-in the dovetails for the front sight attachment. Weihrauch guns that have front dovetails all have this and we have called it a choked bore. It’s really just an accident of the manufacturing process and is as random as can be. But it’s there and some shooters feel it helps accuracy. Even though the choke doesn’t wrap all the way around the bore, they feel that it still provides the same stability that an intentionally-choked bore does.
Here is the lesson
The point is, if a barrel is choked, is it more accurate? The evidence suggests that it is. If that is true, can a choke be added after barrel manufacture? The answer is yes! In fact this may prove to be the most cost-effective aftermarket adjustment that can be made to an airgun.
A choke can be added by rolling the barrel between three precision hardened-steel rollers, one of which is adjustable. By gradually increasing the pressure on the adjustable roller as the barrel is rotated between the three rollers, some compression of the steel is possible. This will affect the inside of the bore, reducing it in size. The worker would have to proceed slowly and watch the progress of the choke, because we are faced with the same problem that the 19th century barrel makers had — namely barrels made from different materials.
This device allows the controlled swaging of a round barrel. The adjustable roller located at 4 o’clock is gradually adjusted inward as the rifled barrel turns.
What we have learned today is that airguns and firearms are very much alike in how their barrels can be made to increase accuracy. I haven’t addressed modern firearms shooting jacketed bullets because they do not respond the same as lead bullets. So in this respect, airguns and black powder arms are the most similar.