I had a friend many years ago that enjoyed black powder shooting. He collected, restored, and shot antique firearms during the 1940s and '50s before most people found it interesting. He had some beautiful, hand-made weapons; Kentucky rifles, English rifles, and similar. During those times, people would very nearly give the things away so he had a large collection.
He told me once about a barrel rifling technique that he called “gain-twist rifling.” The theory behind it went as follows:
Higher velocity is better for a flat trajectory and for impact on the target.
AND
More spin on the projectile is better for accuracy.
BUT
Too high a powder charge causes the bullet to skip across the lands of the barrel rather than engaging them properly, reducing accuracy by reducing the spin and also deforming the slug, reducing accuracy due to poor aerodynamics.
His solution, which he claims to have seen on two occasions, was to vary the pitch of the rifling. For example, at the chamber end, the rifling pitch might be 1 turn in 20 inches, gradually varying to a tighter twist of maybe 1 turn in 10 inches at the muzzle.
(N.B. Those numbers are just pulled out of the air and may not be at all appropriate!)
This sounds like a good idea but I had a couple of questions:
a) Would it actually work in practice to increase accuracy and velocity?
b) Given machine-tools available in the 18th century, could they even do this at all?
c) If this is effective, why isn’t it done today? I’ve thought that the use of jacketed slugs rather than soft lead might explain it but am unsure.
If it ever was done, I would guess that it isn’t done today because there’s little variation in bullet speed from shot to shot with modern manufacturing techniques for both the shell and gunpowder. Any change in the rotation as the bullet travels through the barrel is just going to slow the bullet down and create extra heat. The fewer the changes to the trajectory of the bullet as it travels, the more energy it will have, so if you don’t need to do that, you are better not to.
Sorry, I may not have been clear on this. Admittedly, there is little variation in projectile speed with modern ammunition but higher velocity and higher rate of spin are still seen as good things. (generally) The variable pitch rifling (if it existed) was an attempt to maximize both velocity and rotational speed of the bullet in a controllable manner. The idea was to gradually increase the rotational speed of the bullet rather than instantly going from 0 to some high RPM.
If this technique exists and works, a person could hand-load ammunition to higher velocities and retain accuracy.
Looking at Google, it has been used in the past, and present, for pistols, rifles and artillery. I’d guess it’s a question of whether the results are worth the trouble of a more complex production process.
The new Smith & Wesson Model 460XVR (.460 S&W Magnum) revolver comes with gain-twist rifling.
:smack: Maybe I’m lacking all skills with Google, but I will swear I tried Googling this and came up with 0 hits. On the same subject, have you ever heard of independant testing being done?
It’s not always better to spin the bullet faster. For a mind-numbingly dull discussion, find some AR15 forum and read all about the various twists employed in that rifle - from 1:14 to 1:7. Seems that, in general, modern manufacturing techniques and materials allow us to build to the best spin for the specific bullet size and weight (with some exceptions naturally, thanks mks57).
Yeah, you’re right. The Wiki article that mks57 referenced (thank you again!) had some dope on that. It seems that you can cause them to simply explode if you spin them too fast. Overall, gain-twist rifling seems like a very good idea that just doesn’t work that well in practice.