Twist rate effects on terminal ballistics

[yeti12]

So I have a question for @Formidilosus and others who have a wealth of knowledge on this subject...



Does increased bullet rpm have any effect on terminal ballistics? I feel like if a bullet tumbles in game you probably wouldn't want to spin it faster but a bullet that doesn't tumble it seems that theoretically a little more rpm could cause a little more tissue damage at lower impact velocities. Does a small change make a big difference or would we have to compare massively different twist rates to see a difference?



If this has been covered just link it below, I couldn't find much on the topic. Thanks

 

[Bluefish]

While I am no expert, it seems there are a couple of physical limits based on current technology that limit the range of bullet rpm. To stabilize heavy for caliber, high BC bullets, you have to be pretty fast twist/ high rpm. There is a limit for rpm when using lead core. Seems to be between 300-350k rpm. I have not done the math, but I expect most heavy rounds and speeds expected are between 200k-300k. If we could spin faster, then longer/heavier bullets would be possible.

I know in the Hornady podcast on the eldx, they discussed duplicating stability at long range. They had to twist faster at slower velocity to get accurate results. If they just shot slower speed and not faster twist , the bullets would tumble easier and show better performance than what would actually happen. This suggests to me that slower twists might give better performance if bullet tumbling is the preferred method of transferring energy to the animal. I have gone down a different path and am using big bullets at slow speeds and low rpm due to shooting subsonic. While I have not takes dozens of animals, initial results suggest that has anchored animals more quickly that smaller faster bullets.

If you look at what people claim for extra energy on the 8.6 BO, it’s not much from rotation. That suggests that extra rpm doesn’t have a large effect on energy available.

 

[mother_plucker]

So I have a question for @Formidilosus and others who have a wealth of knowledge on this subject...



Does increased bullet rpm have any effect on terminal ballistics? I feel like if a bullet tumbles in game you probably wouldn't want to spin it faster but a bullet that doesn't tumble it seems that theoretically a little more rpm could cause a little more tissue damage at lower impact velocities. Does a small change make a big difference or would we have to compare massively different twist rates to see a difference?



If this has been covered just link it below, I couldn't find much on the topic. Thanks

Check out the Q podcast with Kevin Brittingham. His 8.6 blackout is shooting 300g solid coppers in a 1-3 twist barrel. The animals he has killed with it, in subsonic and supersonic is impressive. I think that he said they are spinning around 500k rpm. If you look at his Fix in 6.5 Creed, it has a 16" 1-7 twist barrel. He has killed a ton of animals with it, and I have personally shot mine out to 1010 yards pretty accurately.

He has some great videos and images on Instagram and YouTube of bullets after hitting animals.

 

[yeti12]

Check out the Q podcast with Kevin Brittingham. His 8.6 blackout is shooting 300g solid coppers in a 1-3 twist barrel. The animals he has killed with it, in subsonic and supersonic is impressive. I think that he said they are spinning around 500k rpm. If you look at his Fix in 6.5 Creed, it has a 16" 1-7 twist barrel. He has killed a ton of animals with it, and I have personally shot mine out to 1010 yards pretty accurately.

He has some great videos and images on Instagram and YouTube of bullets after hitting animals.

Shorty 6.5 creed with a 1-7 twist is less rpm than a 24" 6.5 prc with a 1-8. I'd be curious to see the differences at same impact velocity with the same bullet. Even a 1-7.5 prc.

 

[mother_plucker]

Shorty 6.5 creed with a 1-7 twist is less rpm than a 24" 6.5 prc with a 1-8. I'd be curious to see the differences at same impact velocity with the same bullet. Even a 1-7.5 prc.

That is true, doing the math on the difference of my 16' 6.5 Creed, and a 6.5 PRC is 14,195 rpm. Now cut down that 6.5 PRC to 18" and the rpm is almost identical to my shorty 6.5 Creed. I'd be interested in seeing some comparisons on actual animal kills, seeing what a bit more velocity does with the increased rpm.

 

[yeti12]

@Formidilosus any experience? Not trying to be a bother but I'm curious if you have seen anything.

 

[Formidilosus]

@Formidilosus any experience? Not trying to be a bother but I'm curious if you have seen anything.

Twist rate (RPM) does effect bullet upset, but it takes a significant change to see it, and usually with rapidly upsetting bullets. Something like going from a 1-12” or 1-14” twist to a 1-8” with 40gr V-Max’s in a 223 or fast 22cal. You will see it with larger calibers, but the effect is less noticeable usually.

 

[Wrench]

That is true, doing the math on the difference of my 16' 6.5 Creed, and a 6.5 PRC is 14,195 rpm. Now cut down that 6.5 PRC to 18" and the rpm is almost identical to my shorty 6.5 Creed. I'd be interested in seeing some comparisons on actual animal kills, seeing what a bit more velocity does with the increased rpm.

A 7 twist @ 2600 is over 260k rpm.

 

[solarshooter]

I had a thought on this the other day when reading through the 22UM thread. We know cup and core bullets can come apart above 300-350k rpm, due to centrifugal force creating too much stress on the jacket - the mass of the core (and jacket) is trying to rip away from the central axis of the bullet. Deep rifling can exacerbate this problem because it creates stress concentrations in the jacket and/or thin spots.

Given this, it seems reasonable that maybe by spinning a bullet closer to it's limit stress, it takes less force on impact with the target to initiate jacket failure and fragmentation. This could manifest as more explosive performance at lower impact velocities.

A further reach would be that maybe this greater centrifugal force also causes fragments to disperse further radially from the central axis of the would channel.

Just some thoughts.

 

[Marbles]

This thread touches on the same topic, might be of interest to you.

Twist rate and penetration: also factors effecting penetration and wound channels in general

So, obviously too slow of a twist will decrease penetration and increase deflection. But, will over stabilizing a quality, heavy for caliber, expanding bullet decrease deflection on bone? That is the question, and feel free to answer without reading more. I'm interested in data/science more...

rokslide.com

 

[mother_plucker]

A 7 twist @ 2600 is over 260k rpm.

yes, at that velocity, my 16" 6.5 Creedmoor with the 1-7 twist MV is 2494. which puts it at 256,525 RPM. Not really sure what you are getting at, my statement was the difference in my rifle and an advertised 6.5 PRC. Using bonded, or solid bullets at faster twist rates is necessary. That 10,903 rpm difference @ 2600fps isn't going to make much of a difference.

 

[huntnful]

My good buddy shoots frangible lead bullets for game animals. He likes Berger hybrids, and he was talking one day that he likes to crunch the twist rate vs. RPM data to keep his RPM’s over 270,000 because he thought they upset better at the higher RPM’s. I’d honestly never even thought about it before, but it does makes sense to me.

 

[mighteywhitehunter]

I am shooting a 1/7 twist 6.5 Creed out of a 21in Barrel with copper solids @ 2850fps and have not seen any difference in wound channels so far. I have also shot critters with the Superformance Factory CX 120gn. No difference terminally, all critters died very quickly.

 

[Timberline001]

I am shooting a 1/7 twist 6.5 Creed out of a 21in Barrel with copper solids @ 2850fps and have not seen any difference in wound channels so far. I have also shot critters with the Superformance Factory CX 120gn. No difference terminally, all critters died very quickly.

Like form stated, it’s going to take a bullet that has rapid expansion to notice a difference if at all. Not a monolithic like those 2.

 

[AkMtnRunner]

With non-frangible monometals, I am curious if there's any downside to higher RPM's. Seems it would only help if there's any difference, except for possibly very slightly lower muzzle velocity at the same pressures.