Home Forums Hunting & Shooting Ask The Gunwriters Twist Rate Impact on Bullet Expansion & Penetration

I’ve got a good pile of 132, 140 and 160 BBCs in 7mm I wanna get to using up. Probably the 132 or 140 in a little Model 725 280 Remington I just snagged. The slower twist in that one should be good with the lighter BBCs but I wished it was an 8 as well

Seems to me centrifugal force would zing bullet fragments off at more right angles to the projectile more than anything else. Could see how this shows more pop on small targets where you want shallow wide as much as possible as folks mention for prarie dogs. For big game seems like would would want the secondary projectiles to travel as far along the bullet path as possible to get in the animals vitals and tear up stuff vs immediately flying at sharp angle upon impact.

Lou

I posted years ago about this topic, always a believer in faster twist, in rifles.....in handguns that twist affected if say a 41 for instance could handle the 'heavies' - accurately. No doubt various handgun mfg. also twist differently in a given chambering.

I would like to know what 'problem' is experienced in the 40s.......was it perhaps 'accuracy related' with 180s?

I've not checked, but are 9mm pretty consistent in twist among various major mfg.?

JWP, you make a good point about fmj. It was noted long ago, that some various military rounds, say a 5.56 and the Russian x45 used twist that marginally stabilized ammo, so it caused more 'yaw' and tumbling of FMJ 'mil-spec' ammo, to 'enhance' wound channel. Now for hunting purposes alone, I would think enhanced straight line penetration, either by depth, or just keeping the bullet nose forward to help ensure reaching vitals would be of value.

No doubt twist affects not only accuracy, but bullet upset, and thereby lethality, in various ways, depending on bullet construction as JB points out.

Very interesting to have media that shows how these effects.

This is a fascinating topic and something I had never even considered. I'd like to see a large scale test done.

Been my experience as well, with non-scientifically conducted tests. Basic Hornady 55 gr. SP in a 1 in 12 pokes a couple holes in a pop can, same bullet out of a 1 in 7 twist comes apart on contact, shredding the can.

End of experiment.

Empty can or can full of water?

Empty

Actually, it is problems.

The two twist rates can make it difficult for an engineer to design a projectile, mostly from a terminal effect but accuracy can also be a concern.


Going from a 1:16 to a 1:10 is a big increase.

Let's look at the 165 grain loading. At a MV of 1150, the 1:16 spins the bullet approximately 51,750 rpm. Change only the twist to 1:10 and it will spin the bullet approximately 82,800 rpm.

Download the round to the, popularly accepted, 1080 fps and you get 48,600 vs. 77,760.

So, as an engineer designing a bullet, do you design for 48,600 rpm or 82,800 rpm?

So much to consider.

It affects all parts of the bullet, whether still attached to the main body or not, and does so along the entire path of travel of the bullet through the animal.

Im just thinking in terms of terminal effect. Spinning something doesnt make a bigger hole. Possibly opposite if spinning reduces frontal resistance through cutting or some effect vs stuff getting blown out of way. I would think for controlled expansion bullets they only expand so much and stop (ie controlled). For bullets that come apart the force should zing fragments away further though more shallow

Lou

For "controlled expansion" bullets, it is when they fully expand. Hypothetically, if it is designed to fully expand in 5" of penetration but a faster twist makes it fully expand in 3" of penetration, a reasonable person could expect its total penetration to be less.

Alternately, if spinning slower than designed (downloaded to simulate downrange), it may not fully expand until 8" of penetration. Total penetration would then reasonably be expected to increase.

You guys can work the math, I'll go by what I've witnessed.

Amen.... give me RPMs!

No rifle hunting bullets that I am aware of require five inches of penetration to expand. Instead most expand fully within their own length, which israrely as much as two inches. The exceptions are "closed hollow points" like the Berger hunting bullets, which penetrate around 2" before the front end collapses and they start to expand.

This is why the area immediately behind and around the entry is most damaged: That's where the bullet totally expands. This has been proven many times in several kinds of media, and duplicated in extensive test-shooting of big game.

My thoughts are that a fragile varmint bullet, as soon as it makes contact and deforms in the least, instantly becomes unbalanced and the centrifugal force rips it apart. That could explain the violent effects on tiny varmints in that it’s not “expansion” in the normal sense, it’s disintegration similar to what is seen with lightweight varmint bullets that disintegrate in midair when fired from the Swift and other higher velocity rounds.

I would think the effect might still affect hunting bullets but would not be as pronounced due to their “stiffer” construction.

There's a vast range of construction in "hunting" bullets. Many will expand completely (and violently) on big prairie dog, even those which penetrate pretty deeply. Have seen this demonstrated with Nosler Partitions many times, due to their relatively soft front core.

Ballistic tips and Sierra TMK do this as well

I lazily chose those numbers of 5 vs. 3 and then 8 in an effort to exaggerate the concept as many would find difficulty believing the rapidity with which modern hunting bullets expand. My apologies.

The concept, however still stands as my opinion. I say opinion as I've not proven the cause. I've merely experienced the results in shots into ballistic gelatine.

I have always speculated that the fast-twist barrels are the reason that 6.5 bullets perform so well on game. They kill way better than the ballistics indicate they should.

sbhooper;
Good morning to you sir, I trust that this second day in December finds you well.

With apologies to those who've read my thoughts on this previously, we've had a couple threads similar to this over the years, one that I initiated myself actually.

Here's a link to it for those interested in what we thought in '14 on this subject.

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

More or less the Cole's notes on that is that I've come to subscribe to the theory that it's the RPM created by fast twist, that can in combination with the correct bullet make a difference in terminal performance. For instance the loads I talked about in the above thread would have had the Swede with a projectile RPM of 234,000 and the unusually slow twist .270 making 190,000 RPM.

We can all agree that "dead is in fact dead" when it comes to animal reactions, but depending upon where we hunt, sometimes dead in close proximity is a very good thing, you know?

Two recent video examples I was viewing were a Tahr hunt in New Zealand and a Dall Sheep hunt in the Yukon where the hunters were both heard imploring the downed animal to please stay right there!

As always, there's many roads to Mecca, though of course with the beer flu they're not very packed these days one can imagine.

All the best to you as we head into colder and shorter days.

Dwayne