Home Forums Hunting & Shooting Ask The Gunwriters Does twist rate affect muzzle velocity???

Assume I have a load worked up for the .38-55 and its 1-in-18" twist barrel. Now, were I to fire this exact same load in a .375 Win. with its 1-in-12" twist barrel, would there be any decrease in muzzle velocity as a result? If so, how significant?

Thanks for any input.

RSY

Twist has no significant effect on pressure or velocity. I don't think the difference would even be measurable in a true test. The situation that you describe would include other variables that would have to be matched or eliminated for the test to be a valid test of the effect of twist on velocity.

Not that Ken needs any support on this but a couple of fellows(Vaughn & McCoy) that made a living dealing with ballistics also wrote a few books on the question. Their take is less than one half on one percent of the energy generated by a cartridge is used to impart the spin on a bullet. So I suppose that a difference in twist of 2" or so would pretty much drop that down into the 1/100th's of a percent range. The other variables are going to account for more deviation in velocity.

Doubt the answer could be quantified in the best lab in the country.

I once asked an uncle of mine, who is a nuclear physicist, about the amount of energy that might be imparted to a living, three-dimensional target due to the rotational velocity of a bullet. His reply was that it would be very, very insignificant.


-RR-

That sounds odd to my logic. It would seem that the faster a bullet is spinning, the more it would distort, resist, expand, and release energy while going through game?

RSY ... I was thinking about this same thing the other day ... I was trying to think of a mathematical relationship to explain it .. guess I don't have to now ... <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

Considering a whitetail is roughly 12-in. thick, a 1 in 9 twist would rotate the bullet an extra one-third over a 1 in 12 twist. In other words, the faster twist would rotate 1.3 times to the slower twist's 1.0 on the path through the deer.

That's not a lot of difference.

I have read from some expert somewhere that the spin on the bullet assists with the mushroom forming as the bullet enters the body. That makes sense to me since the rpm's are so high. I can imagine that once the bullet begins to deform and flatten out that the bullet's rpm's would assist and even amplify the mushroom's diameter. That expansion of diamer of the bullet certainly does impart additinal destructive energy into the target from the projectile.

For a difference of a 2 inch twist to even 3 or 4, probably not significant.

However, if you really speed things up (like going from a 1:14 to a 1:8.5 I think you're going to see some measurable pressure increase.

An old engineer's trick is to exaggerate the effect to give you an idea what the outcome of the change would be. Imagine a 1:10 twist 30-06 being changed to a 1:4 twist! Don't tell me you wouldn't see a pressure spike there <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> ! So, there will be some effect on pressure.

Dan

No effect on Muzzle velocity at all, however with thin jacketed bullets the combination of high velocity & fast twist can result in bullet fragmentation in flight. this is due to extreme rotational velocity exceeding the ability of the jacket & the core to hold together ( note Hornady makes a .22 cal bullet that they advise not to use over 3000 fps in a 1x9 twist barrel). as for the effect on hittiing living targets [terminal ballistics], the rotation makes no difference at all, in fact I would be readily convinced that rotation on the original axis ceases on contact as the projectile will yaw off the original line of travel & begin to "tumble"
I am sure there are more accurate "scientific" terms to describe this but what the heck, I'm just a dumb harley ridden, knuckle draggin gun packer.