by Tom Gaylord
Writing as B.B. Pelletier
The history of airguns
This report covers:
- Fine rifling
- The Trapdoor barrel
- Bullet deformation is bad
- Pope’s muzzleloading breech loaders
- Dr. Hudson
- Airguns and rifling
- Twist rate
- Diabolos and twist rate
- One last question answered
I am running this report immediately after the Part 1 because of the questions several readers asked. I can see that the subject of rifling is not understood that well. I stopped the first report with the invention of Ballard rifling, which I said was shallow thin lands and wide grooves. Today I will start at that point.
Ballard rifling was great because of what it did — or rather what it didn’t do. Ballard rifling did not deform the bullet as much as the other kinds of rifling I mentioned last time. You will recall that I said the development of the Trapdoor Springfield rifle marked a major advance in the development of ammunition. It did not do the same for rifled barrels.
The U.S. Rifle model 1873 is best known as the Trapdoor Springfield.
The breech is closed.
And this is where the name Trapdoor came from.
The Trapdoor barrel
The Trapdoor barrel has three lands and three grooves of equal width. That means the lands are very wide. They are also 0.005-inches high. That’s fairly shallow for a .45 caliber rifle barrel, but the width of the lands is a problem for ultimate accuracy. The lands shave a lot of lead off the bullet as it traverses the bore. That’s why when the government developed a long-range rifle on the Trapdoor platform they gave the barrel six lands and grooves. The standard twist rate of one turn in 22 inches for the 405-grain government bullet was increased to one turn in 19-5/8-inches to stabilize the heavier 500-grain bullet fired in the long-range rifle. More on twist rates in a bit.
Bullet deformation is bad
It turns out that the more a bullet is deformed by the rifling, the less accurate it will be. That was what was so revolutionary about Ballard rifling. It didn’t deform bullets as much as most of the rifling that preceded it. It was very good, but it wasn’t the absolute best.
The deformity I refer to is in the form of lead “fins” left on the base of the bullets by the lands after the bullet exits the muzzle. Harry Pope believed these fins interacted with the exiting gunpowder gasses, causing slight instabilities at the muzzle of the gun.
You can see how the rifling lands have left lead fins on the base of the bullet at the right, after it has traversed the bore. And this bullet went through a barrel that has Ballard rifling! According the Harry Pope, this deformity causes inaccuracy because of the interplay with the powder gasses and the irregular bullet base during the first few inches after the bullet leaves the muzzle.
The best rifling was made by Harry Pope. Pope put 8 extremely narrow and low lands in his barrels. He understood that the bullet base shouldn’t be deformed. And then he did something else — something that made his bullets (and barrels) the most accurate ever made, up to that time. He made his rifles load the bullets from the muzzle and the cartridges load from the breech!
Pope’s muzzleloading breech loaders
Pope believed that the base of the bullet was critical to accuracy. He knew that rifling left lead fins around the base of each bullet fired, so he made rifles that loaded from the muzzle. There were fins of lead even with his extremely narrow and low lands, but because they were loaded from the muzzle the fins were on the front of the bullet, where they didn’t affect the accuracy.
To load a Pope rifle you first loaded a dummy cartridge into the breech. This cartridge had a plug that extended 1/10-inch into the bore past the end of the case. Then you loaded the bullet from the muzzle. It stopped when it came against the plug in the dummy cartridge. Then you removed the dummy cartridge and replaced it with a loaded cartridge. The rifle was now ready to fire. Pope got such superior accuracy with his barrels and method of loading that he was backed up for many years on barrel orders.
Around the turn of the 20th century, another fine shooter named Dr. Hudson decided to change shooting forever. He breech-loaded his bullet and pushed it into the rifling with a mechanical device called a bullet seater. The seater pushed the bullet into the bore 1/16 to 1/10-inch deeper than the cartridge would go. Dr. Hudson used bullets with rings at the back that were just slightly larger than the bore of the rifle. These bands guaranteed sealing against the gasses, and left the rest of the bullet untouched by the rifling.
This bullet seater just slides the bullet deep into the rifle’s chamber. This is used in a rifle whose breech block pushes the dummy cartridge into the chamber to push the bullet into the rifling.
This lever-type seater mechanically shoves the lead bullet into the rifling of the bore. It’s used on rifles whose breechblocks do not lever the dummy cartridge forward. That’s what this device does.
All of these special loading methods are designed to extract the absolute finest accuracy possible from a rifle. They are not for regular shooters. They are for fanatics. But they showed riflemen everywhere what the finest rifling looked like and what it could do.
Airguns and rifling
The first modern air rifle was made by the BSA company in 1905 — right at the time all these important rifling lessons were coming together. This was decades before Marlin’s Microgroove rifling had been thought of and decades before polygonal rifling was tried. But BSA was aware of one thing from the start — light lead pellets are difficult to launch with just a small puff of air. Anything that inhibits their movement — like deep rifling or wide lands, is a problem.
So from the beginning airgun rifling had low thin lands. The diabolo shape of the pellet already stabilized it in flight to some extent, so the spin from the rifling was just gravy that added more range to the pellet.
The twist rate determines how fast the bullet or pellet spins in flight. Longer heavier bullets need to spin faster for a given caliber. But a large caliber bullet, say .75 caliber, needs less spin to stabilize than a small caliber bullet. If you think of the planet Earth as a large ball, it spins once in approximately 24 hours and is reasonably stable.
Back when round balls were the only bullets around, the twist rate of the rifling wasn’t so important. The Hawken brothers popularized a 1:48 twist (one turn of the rifling for every 48-inches of barrel length), but that was simply because that was the only twist rate their rifling machine could rifle! It wasn’t that calculated. It happened to work, but there were rifles on the market at the same time that had 1:72 and even 1:90 twist rates. Today you’ll find shooters who think 1:48 is magical because the Hawkins used it, without knowing (or caring) why they did.
When the change was made to conical bullets the twist rate became critical. A conical bullet needs to spin on its long axis to be stable, and the twist rate has to be calculated precisely to do that. My AR-15 has a 1:8 twist rate and stabilizes 69-grain bullets perfectly. Shorter 55-grain bullets spray all over the place — the length of the bullet is that critical.
Diabolos and twist rate
But airgun pellets are projectiles with high drag. They are not entirely stabilized by spin. In fact most of their stabilization comes from the aerodynamic drag on their tails. So the rate at which the rifling spins them is far less critical.
Early airgun barrels were made by BSA and were heavily influenced by the firearms they made. They used the 1:16 twist rate of a .22 long rifle cartridge and applied it to their airgun barrels. And just like the Hawkin Brothers before them, they used the same rate of twist on all their barrels, regardless of the caliber. That practice continued with all other airgun makers, pretty much through the rest of the 20th century.
Where a .22 rimfire bullet performs differently depending on the barrel twist rate, pellets aren’t nearly as sensitive. Here is how critical it is for bullets. A .22 long rifle (shooting a 40-grain lead bullet) barrel twist rate is 1:16, but a barrel made specifically for the .22 short ( a 29-grain bullet that’s much shorter) has a twist rate of 1:20 or even 1:22. For guns that shoot both cartridges, the long rifle twist rate is used. But with diabolo pellets the twist rate isn’t as important. So for a century, 1:16 worked well.
Is 1:16 the absolute best for all pellets and all calibers? Maybe not. But not very much research has been published about what works and what doesn’t. Barrel makers probably know more than they are telling, though.
One last question answered
Reader Matt61 said this, last time.
This is great. The first question that occurs to me is one that I’ve had for awhile and may seem trivial, but it nags at me. When the bullet “catches” the rifling, does that mean that the rifling cuts into the bullet to a depth equal to the height of the land + the depth of a groove? Cutting even that much into the surface of a bullet seems like a lot.
Matt, the “height of the land” IS the depth of the groove. They are not cumulative. They are the same. Metal is cut away to form a groove and the land is the metal that remains.
How does the bullet “catch” the rifling? I asked you to tell me how it worked when the bullet was loaded from the muzzle. When the bullet is loaded from the muzzle, it is forced into the rifling by hand and pushed down the bore until it stops. The forcing is what causes the rifling (lands) to “cut” into the side of the bullet. Or, if you shoot a patched ball, the cloth patch gets jammed into the grooves, leaving the edge of the round ball untouched by the lands. When the patch spins from the rifling, it spins the ball that’s inside.
A breech-loaded bullet does encounter the rifling when it fires. Or, if you push it into the rifling during loading like I have shown here, then you are the one who pushed the bullet into the lands. In many rifles, the lands are cut on a taper ahead of the breach. This allows the bullet to be engraved gradually as it moves forward. I hope this answers your questions.
I haven’t talked about things like gain twists (where the twist rate increases as the bullets advances down the bore) and choke-bored barrels. I have discussed them in the past, as you can see in the linked report. And I will mention them again in this series.
In the next report on rifling I will concentrate on airgun barrels. I’m not going to do that as the next history report though, because not everyone is equally interested in this subject. So I’ll give it a bit of a rest. But I will come back and tell you more about airgun barrels and rifling.