Is There a Relationship between Twist and (a) Velocity and (b) Pressure?

[South Pender]

I've read on another forum the opinion that a faster twist results in higher pressures than a slower one. I've also wondered whether a faster twist might reduce velocity. The latter sort of seems sensible as you'd have to think that some of the energy propelling the bullet would be used up in driving the bullet's rotation, and with a faster twist, there's more resistance on the bullet than you'd have with a slower twist. But does this affect muzzle velocity?

Consider the following experiment. You load up some cartridges--let's say 6.5 Creedmoor--and fire them in two different rifles, both having identical chambers and throats and identical bore and groove diameters:

(a) one with a 10" twist and

(b) a second with 8" twist. (Assume that the bullet weight and length permit stability in both barrels.)

You seat the bullets to have a .050" jump into the rifling in both. So there's no difference between the two in terms of the behavior of the bullet before it hits the rifling.

Will the velocity be the same in both? Will the pressure be the same in both?

 

[calgarycanada]

Bryan Litz tested this and found very small, almost negligible effect. So faster twist will create higher pressure and slower velocity on paper but not something you need to worry about.

 

[fkimble@charter.net]

Since two identical barrels may not produce the same velocity, this will be very difficult to prove decisively.

Frank

 

[Fuj]

I'd be more worried about torque as the twists get faster.

 

[SKWERLZ]

The pressure difference between twists is really inconsequential.

Jim boatright did a paper on this with the calculations.

 

[ronemus]

Since two identical barrels may not produce the same velocity, this will be very difficult to prove decisively.

Frank

Bryan put serious effort into his tests - 10x replication IIRC.

 

[fkimble@charter.net]

I'm sure Bryan did a very good job. My biggest concern ref twist is a rough throat damaging the jacket, then the over twisting heating the jacket and lead, then centrifugal force destroying the bullet en-route to the target. Had a brand new 223 barrel (8 twt) that was poofing H 55fmj. After one practice session, it stopped doing it. I assume it had very sharp corners on the rifling when brand new?

Frank

 

[Nealm66]

Does the faster rotation add bc to the bullet?

 

[dstoenner]

Does the faster rotation add bc to the bullet?

No. It just gets the most of the inherent BC it can get.

David