Home Forums Hunting & Shooting Hunting Rifles Does anyone think a faster twist helps with terminal performance?

The thing is the velocity difference due to twist is lost in the noise.

I maintain the difference in terminal performance on game - going from a nine twist to an eight twist - is akin to the difference in terminal performance going from a 280 to a 280 ai. Is there a difference - sure. Is it meaningful … ?

The 12 twist to 7 twist example is probably a bit more significant.

The actual difference in velocity between a fast twist and a slow twist is a little more when pressure is kept constant. I believe using the same load doesn't keep pressure constant, it increases pressure in the fast twist which tends to increase velocity, partially cancelling out the greater loss from a faster twist rate.

Yes this is true, same as the difference between a .270 and .280 etc. Nevertheless its interesting to know whether it makes any difference at all or absolutely zero.

all things equal, I dont see why a faster twist WOULDNT impart more movement into the game.

that's why you spin a bowling ball - to impart "pin action" caused by that rotation, imparted from the ball into the pins.

that said, the ballistics of a bowling ball and a bullet are obviously radically different. interesting to think about, just the same.

Riflehunter;
Good evening to you sir, I hope the week was a fine one for you and you're well.

We've asked this question a few times here on the 'Fire over the years and I do mean to include myself in the "we".

This is from 2014.

https://www.24hourcampfire.com/ubbt.../tissue-damage-270-vs-6-5x55#Post9248055

In the years since that thread I've seen nothing that makes me change my mind, which is to say that I do believe that additional rpm do help certain types of bullet construction to work if not better, than more reliably.

All the best whichever twist you choose to use.

Dwayne

Like anything else it depends. Many long treatises have been written about this.

BTW this thread rocks.

Well thank you Dwayne, you are a true gentleman...I read the whole thread. Now I'm glad my .270 has a 1 in 9 twist, even though all the bullets I use only require a 1 in 10. I just need to determine whether I should go for a 1 in 10, 9 ,8 or 7 for a .257 with the intention of only using 110s which currently need only a 1 in 10.

I would say that physics with its models and results are more than a little relevant to understanding how things work, in the case of ballistics as much as anything.

How difficult is it to use the 9.3 at 400-500yd distant? I've done years with the 308win and recently the 270win and big 338's but am curious about 9.3

BTW, here's that post I mentioned.

https://www.24hourcampfire.com/ubbt...m-compared-to-bullet-weight#Post16991752

I'll paste the text here:

"Okay. A few comments on the physics here.

First, in terms of the interior ballistics, let's assume a case in which we have a .224" 80 gr bullet being accelerated to a MV of 3000 fps, and the barrel has an 8" twist. For simplicity, I've approximated the bullet as a cylinder with a cone in front of it, where the cylinder and cone have equal lengths. In that case, the bullet leaves the muzzle with 7.5 joules of rotational kinetic energy and 2167.2 joules of translational kinetic energy. In other words, the rotational energy is 0.3% of the total kinetic energy, and the "forward" translational energy is 99.7%.

Second, the effect of angular velocity on terminal ballistics can be divided into two distinct cases: FMJ bullets, and expanding bullets.

In the first case, the mechanism by which FMJ bullets damage tissue is by de-stabilizing and tumbling. I haven't calculated the destabilization of the bullet due to the laminar drag of the tissue, but I suspect chaotic behaviour, so the effect of increased rotational speed isn't clear to me. It seems logical that the more stable a bullet is when it enters tissue, the less likely it is that the bullet will begin tumbling immediately, but the difference may be negligible when impacting non-uniform tissue and bone.

In the second case with expanding bullets, I'll approximate the bullet as a cylinder. When it hits tissue, I assume that the bullet expands to twice its original diameter. I also assume a conservation of angular momentum. Here we see that the bullet loses 70% of its angular velocity upon expansion, and 75% of its rotational kinetic energy, simply due to the increase in its moment of inertia. The rotational kinetic energy goes from ~8 joules to ~2 joules. So not enough of an energy difference to do much work on the tissue by itself. In reality, angular momentum is not conserved, since the tissue applies a torque on the bullet in the form of drag force causing the bullet to slow down even more, and consequently the bullet applies a torque back on the tissue, causing the tissue to accelerate radially from the bullet's major axis.

The real difference in terminal effects between different twist rates is seen in the centrifugal force on the core and jacket of the bullet. Assuming the same 0.224" 80 gr bullet going 3000 fps, fired from an 8" twist barrel versus a 14" twist barrel. I'm also assuming the mass of the bullet is concentrated in the jacket, for simplicity (this leads to an inflated value for the centrifugal force on the jacket, compared to when the mass is distributed uniformly throughout the bullet's diameter). The bullet fired from the 8" twist barrel experiences 11,790 newtons of centrifugal force on the jacket, while the bullet from the 14" twist barrel only experiences 3,850 newtons, or about 1/3 of the force trying to rip the jacket apart. This is equivalent to about 2650 pounds of force on every point of the bullet jacket, versus 865 pounds. The additional force would have a significant effect in expanding and fragmenting the bullet, and depending on the design of the bullet, this could have a meaningful effect on tissue damage.

One more thing to add: with respect to interior ballistics, if instead of an 1:8" rate of twist we had 1:14", the rotational energy would go from 7.5 to 2.5 joules, or 0.3% of the total kinetic energy to 0.1%."

"Definitely Not".

Thanks for posting that Jordan.

Riflehunter;
Good evening once more sir, thanks for the reply and kind words it contained.

I see that I wrote "if not better, than more reliably" and meant to write then, but if that's my only typo then I'm doing quite well!

The other day I was listening to a YouTube video discussing the new 8.6 Blackout where they're using a 1:3 twist, so the boundaries are being pushed more every day it seems.

There has to be trade off such as mathman mentioned with some loss of velocity.

Since that thread I've played around a wee bit with this online projectile RPM calculator.

http://www.findnchoose.net/bullet_rpm_calculator.html

The slower bullet from the Swede was turning 69,316 rpm faster than the .277 bullet from the slower than usual twist .270 and perhaps that was the cause of the difference in tissue damage? Perhaps?

To be sure we were looking at carcasses so it wasn't like the bullets didn't work at all but we did see some differences and wondered what the cause might be.

I'll note that my late friend BobinNH was a firm believer in 1:9 .270 barrels and he used them a fair bit, so that's a good foundation.

When I was still fooling with buddy's B78 in .25-06 we never measured the twist and the only other .257" barrel I've played with to any extent has what I believe is a gain twist barrel made by a chap in Alberta years back. I'd have to measure it again, or better said attempt to do so to recall what I thought the final twist was nearer the muzzle.

With the bullet technology seemingly growing in leaps and bounds every year, I'd think that depending upon case capacity a least a little extra faster twist can't be a really bad thing.

Gentlemen such as my friend Jordan are able to explain the math and physics many times better than I'll ever be able to do for sure. My observations are that of a student only - perhaps a serious student, but still not someone with many answers, just a bunch of questions usually.

All the best and thanks again.

Dwayne

Jordan;
Good evening my friend, I hope the day was decent to you all on your side of the big hills.

I was happy to hear and see on the news that they're going to do a real Stampede again this year, I hope that it goes swimmingly and some of the businesses get a much needed financial boost.

Thanks for the post sir, I've read it twice and I "think" I'm starting to wrap my head around it all.

When we've chatted about this before, I believe I've said it's more what the greater rpm does to the projectile rather than the rotational energy difference.

Your post surely explains what the Swede did to thin jacketed bullets - coupled with the fairly deep grooves in the barrel it's a recipe for them to come unglued.

Someday it's my hope and prayer we get a coffee or a meal together Jordan. If you ever make it out west here and have the time, please feel free to drop by.

All the best and thanks again.

Dwayne

Thanks, Dwayne! The same goes for you if you happen to be on this side of the hills.

Yes, I think you’re right about that. My sense is that it’s less about the ratio of rotational kinetic energy to translational kinetic energy, and more about what the angular velocity does to the bullet itself (as evidenced by some of the photos in this thread), which changes the way the translational kinetic energy gets transformed into mechanical work done on the tissue.

This makes sense to me. The wound is proportional to the KE. Playing with twist, bullet, constructiom, etc is shaping that wound. In the case of fragmenting bullets for ex, faster twist shreds the bullet a little faster allowing for more energy transfer in shorter period (i.e. pop a ground squirrell). May or may not “improve” terminal effect depending on what you want to do just like any of the other variables involved

Lou

Well then post a proven model.

Just sayin.

Your "sense" is pretty far from an actual proven model.

Just Sayin twice.

Obviously you are free to post guesses and "sense" but don't pretend you have a "model" that accurately predicts the terminal effect of twist rate.

Burnsy's got a hard-on for Physicists. It's hilarious.

Not all of them.

Just the ones that don't know that laminar flow does not happen in terminal bullet performance. EVER.

Turbulent flow does happen in terminal bullet performance every single time.

He is litterally spouting pure Bull Schitt.

Just sayin.