PDA

View Full Version : progressive or non-linear rifling


fisherman66
March 7, 2008, 10:14 AM
Has there been a rifled barrel maker that has made a tube with rifling that starts with a slow twist and that gets progressively faster as a bullet approaches the muzzle? Would this allow for a small increase in FPS and the ability to launch more fragile varmint style bullets?

Unclenick
March 7, 2008, 10:42 AM
The old Gain Twist Barrel Company did, but they folded up. I don't recall who did it first, but the idea has been around for some decades. AFAIK, it has never proven to help bullets or accuracy. I think the idea has two problems: First, it ignores that the bullet is accelerating in the barrel anyway, so increase in rotational velocity occurs at the same rate as for the bullet's velocity in linear rifling, anyway. Second, Harold Vaughn has shown that core separation and slip is a real phenomenon when you give a bullet too much rotational acceleration. Once the jacket gets spinning faster than the core, after exiting the barrel it is slowed by friction with the core with the result you get less net stabilizing spin than the twist rate should give you. You also get a jump as the masses align on a sometimes-new axis, so accuracy is deteriorated. If you leave the majority of your rotational acceleration of the bullet for the far end of the barrel to accomplish, the bullet is already going pretty fast when it gets to the more rapid twist, so peak rotational acceleration is increased. That results in core separations at a lower muzzle velocity than would occur in a conventional barrel.

A non-linear twist function could be devised to make rotational acceleration truly constant, I suppose. It would be different for every powder, charge, and bullet, however, which makes it a difficult thing to apply in practice.

fisherman66
March 7, 2008, 10:56 AM
Thanks for the response.

"Harold Vaughn has shown that core separation and slip is a real phenomenon when you give a bullet too much rotational acceleration. "

My initail thought is the jolt from a millimeter or so of freebore to 1:10 (or whatever twist) is enough to cause too much rotational acceleration for fragile bullets. A more gentle accelerating twist might actually solve that problem, but I see your point that since the bullet is accelerating the initial millimeter is the slowest speed and the last couple inches of barrel would apply the strongest rotational force.

Unclenick
March 7, 2008, 11:23 AM
I am theorizing there. I just know the barrels never proved really popular. They were said by some to be best for cast bullet shooting, which is lower velocity, and may, indeed have been their best application. I never tried one. The cast bullet connection suggests the idea may go back to Harry Pope, or one of the other old time master riflesmiths.

When running pressure computation in QuickLOAD, I find most rifle bullets using conventional rifle powders are under peak pressure a couple of inches forward of their starting point. For constant acceleration, you want the rifling to be an inverse of the pressure vs. barrel length curve, or better, still, to include barrel friction, you'd actually want to use the first derivative of velocity in the curve below. Note that QucikLOAD sees barrel length the way the BATFE does, starting at the boltface, so the bullet travel begins from its location forward of the boltface.

http://img169.imageshack.us/img169/7345/3006175grainssierra2wr3.gif

Anyway, mirror that pressure curve around the x-axis. That's about the shape of the rifling rate curve needed for constant rotational acceleration, except for the freebore and jump part would need tweaking with a very short transition from straight to twisting. It would start, after that short transition, with more twist than it has at the pressure curve peak, and increase in twist rate again afterward.

fisherman66
March 7, 2008, 11:50 AM
Thanks for the graph. It really addresses the meat of the issue.

Forgive my ignorance, but PSI and Copper Units of Pressure are not the same correct (are they even "coorelateable")? Does your software have a CUP option in lieu of PSI?

I think the benefit of a non-linear rifled barrel is that it might flatten the curves (especially the pressure) and allow for faster powders and perhaps shorter barrels without sacrificing effiency.

Interesting stuff. Thanks for the conversation.

brickeyee
March 7, 2008, 02:10 PM
Gain twist rifling is going to change the burning performance of the powder IF it creates any change in pressure.
The burning rate of smokeless powder is a function of pressure (and pressure in a closed vessel is a functionof the burning rate).
The maximum pressure is NOT always coincident with the bullet engraveing into the rifling.
Gain twist rifling also may casue blowby as the engraved area on the bullet is no longer uniform, but is going to exhibit widening as the engraving is made to occur the entire length of the gain twist.

Jim Watson
March 7, 2008, 02:28 PM
Gain twist barrels were made by
Sam Colt for cap and ball revolvers, perhaps to gradually spin up a round ball with short bearing surface.
Harry Pope for scheutzen target rifles, because he considered it helped accuracy. Hard to argue with that, but Schalk, Schoyen, Pedersen, et al did not bother and they made some very accurate rifles, too.
Salvatore Carcano, for the Italian service rifle, perhaps to reduce the resistance of spinning up the long skinny 6.5 mm bullets with the newfangled smokeless powder.
Smith & Wesson for the X-frame monster magnums, likely to gradually spin up revolver bullets without shredding them.

fisherman66
March 7, 2008, 02:49 PM
So it's "gain twist rifing".

"Gain twist rifling is going to change the burning performance of the powder IF it creates any change in pressure."

It seems that some sophisticated internal ballistic software should be able to model a barrel that would take advantage of that and offer a higher muzzle velocity by maintaining peak pressure over time with less stress on the system.

"Gain twist rifling also may casue blowby as the engraved area on the bullet is no longer uniform, but is going to exhibit widening as the engraving is made to occur the entire length of the gain twist. "

That makes me believe only very ductile bearing surfaces could be used, perhaps negating the benefits of a more gentle twisting of highly frangible bullets which are typically thin jacketed.

James K
March 7, 2008, 03:29 PM
FWIW, I have never seen convincing evidence of any advantage to gain twist that outweighs the extra trouble and cost of making the barrel. Also, I have never seen anything on the effect of the rifling on the bullet engraving (skid marks) and what effect, if any, the air has on it.

What I have seen is either anecdotal, with no comparison or means of verification ("I got this great rifle and it shoots great because it's got this great rifling") or advertising hype from the few companies that have made such barrels.

It is possible to have a barrel made with gain twist rifling, but I am inclined to think it would be a waste of money.

Jim

brickeyee
March 7, 2008, 04:08 PM
It seems that some sophisticated internal ballistic software should be able to model a barrel that would take advantage of that and offer a higher muzzle velocity by maintaining peak pressure over time with less stress on the system.

While it can be modeled, if you look at the pressure curve you see constantly falling pressure after the peak as the volume of the gasses is increased with bullet travel.

The amount of energy required for spinning up the bullet is a very small fraction of the energy required to simply accelerate the mass of the bullet.

While it is the area under the pressure time curve that determines the final velocity, it is the peak pressure that limits how much powder can be used.
Lowering the peak but increasing the area has been a highly sought after thing for many years.
Gain twist rifling is unlikely to have a significant affect in altering burning rate pressure curve shape since the energy of rotation is actually rather small.

5whiskey
March 7, 2008, 04:17 PM
like shooting a bb gun at a freight train

Zak Smith
March 7, 2008, 04:47 PM
These guys talk about using gain twist barrels from Lother Walther for very high BC bullets in .338 Lapua
http://www.lima-wiederladetechnik.de/Englisch/LM-105-long-range-bullet.htm#LM-105

davlandrum
March 7, 2008, 06:17 PM
The SW .460 uses a gain twist rifling.

Sorry if this is "apples to oranges".

The graph made me realize I have no idea how it works (the graph, that is :p)

What benefit it provides, I have no idea, but it makes it sound cool.

Shoots pretty good, as well..

fisherman66
March 8, 2008, 08:56 AM
Does the S&W recommend a certain type of bullet for the 460? Have you even been able to recover a bullet to examine the jacket?

I guess the current tech direction to keep pressure high for an extended period of time is dual burn speed powder charges. I remember reading about someone pulling out the "light magnum" charge of a Hornady cartridge. I don't recall if there's a core of fast powder inside a slow burning charge, but that's how it would make sense to me.

davlandrum
March 18, 2008, 12:05 PM
Fisherman,

Sorry to not check back on this for awhile.

While SW does not recomend a specific bullet, it seems like it was made for the Hornady .460 commercial ammo. It is a light round (200 gr) getting pushed way fast for a revolver (2,000 +). With this load, a zero at 150 yds will keep you in kill zone at 200.

I was worried about shooting lead .45 Colt out of it because of the twist. I thought it might lead up fast, but that does not seem to be a problem.

I have not recovered a bullet so can't give any info on what it does to the jacket.