Rate of Twist


PDA






pohill
December 6, 2012, 04:40 PM
I'm looking at a reproduction of an 1860 .54 Gallagher. This repro has a barrel twist of 1:18. The originals were 1:72 (in .52 caliber). I read that with a fast twist you should use harder lead or the grooves will fill up from soft lead and you'll have a smoothbore.
The repro Gallagher uses a cartridge with either a .540 bullet or a .535 roundball. The bullets or balls are hand/finger pressed in - no seating dies involved.

So, this is what I'm trying to wrap my brain around.
I'm thinking that with a slow twist (1:72) the ball or bullet doesn't have as much of a chance of engaging the rifling as in a fast twist (1:18). Or is that backwards thinking - the slower twist rifling actually has a better chance of grabbing the bullet/ball.
And, wouldn't softer lead engage the rifling better?
Any ideas or info?

If you enjoyed reading about "Rate of Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
woodnbow
December 6, 2012, 06:20 PM
"I'm thinking that with a slow twist (1:72) the ball or bullet doesn't have as much of a chance of engaging the rifling as in a fast twist (1:18). Or is that backwards thinking"

Yes, that's kind of backwards. In a slow twist barrel there will be less rotational speed than in a fast twist, velocity being equal. Thus less resistance to the spin imparted by the rifling.

"the slower twist rifling actually has a better chance of grabbing the bullet/ball."

This is true in this sense; if you drive a long bullet (long bearing surface etc.) thru a fast twist barrel it will likely not strip the rifling and it will be stabilized by the faster rate of spin imparted to the bullet. If you were to attempt to drive a round ball at a similar velocity thru that same fast twist barrel it might not have enough bearing surface to be gripped by the rifling and would thus "skip" the rifling resulting in an unstable, inaccurate projectile. The idea is to have a high enough rate of spin to stabilize the chosen projectile, round balls require less "spin" than bullets and large bore projectiles require less spin than small bores.

"And, wouldn't softer lead engage the rifling better?" yes until you reach a velocity at which it wants to skip the rifling...


Any ideas or info?" If that were my rifle I'd load it with a soft lead bullet for longer bearing surface and use small powder charges.

pohill
December 6, 2012, 06:50 PM
The bullets and roundballs appear to be about the same size. Does the short length of the barrel - 22" - come into play with the 1:18 twist?

The Gallagher bullet is listed after the Sharps in this chart:
http://www.castpics.net/subsite/CurMolds/Rapine.pdf

woodnbow
December 6, 2012, 06:56 PM
Looks like that bullet has at least twice the bearing surface of a round ball taking into account the grease groove and all... 1:18 is pretty fast twist for .54 caliber, I'd use the bullet. The length of the barrel won't affect any of this to any great degree.

pohill
December 6, 2012, 07:13 PM
From what I've read the Erma repro barrels were made from German machine gun barrels - they could get two Gallagher barrels from each machine gun barrel. It seems that many Gallagher repro owners have the barrels relined in .50 caliber.
It looks like an interesting gun to shoot.

woodnbow
December 6, 2012, 09:36 PM
It does, if it's accurate it's even more interesting!! :)

pohill
December 6, 2012, 10:21 PM
Most shooters say they're accurate to 50 yds, but relined in .50 or .52 with a slower twist, they're good out to 100 yds.
I'm still not sold on it yet.

If you enjoyed reading about "Rate of Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!