Mule Deer,
Are you aware of any bullet tests where the only variable is the rifle twist rate? Same bullet at the same velocity shot into test gel or other material, just one from a 1:11 and the other from a 1:8, for example?

I am curious to how much of an impact rotational velocity has on penetration and bullet expansion. Is the success on game of these new long, heavy, high BC bullets in the sectional density, or does the twist rate contribute, too?

Bill Steigers from Bitterroot Bullets seem to think twist rate had a significant effect.

How could twist rate have much of an effect? A twist rate of 1:12 means that the bullet only makes about one revolution inside a deer.

bonepoint,

I made once such a "test" eight years ago, when trying out some new 50-grain .224 plastic-tipped varmint bullets on prairie dogs. My shooting partner and I had two primary rifles, a traditional .220 Swift with a 24" 1-14 twist barrel, and an AR-15 ..223/5.56 with a 20" 1-8 twist barrel.

Obviously, the Swift's muzzle velocity was considerably higher, around 3800 fps if I recall correctly, and the AR-15's around 3100 fps. The AR "exploded" PDs noticeably more than the Swift all day long.

Twist tends to have less effect with controlled-expansion or even cup-and-core big game bullets, but does indeed affect expansion, due to centrifugal force.

It’s not the forward motion, it’s the increase in inertia energy stored in the bullet, which, it is worth noting, does NOT decrease near as fast as the energy from the forward velocity. The rotational energy is partially bled off by the deformation of the bullet upon impact.

Dutch,

I also happened to get in on the ground floor, thanks to knowing the guy who developed it, of a new media for testing bullets. This was a wax which was just enough "harder" than ballistic gelatin to retained the dimensions (and other details) of the "wound channel.".

Among the items retained by the wax was an impression of the rotation of the bullet. One interesting thing is that the channel produced by so-called "premium" lead-cored bullets almost always reflected the twist-rate of the barrel. If the twist-rate of the barrel was, say, 1-12 inches, then the hole through the wax beyond the initial bullet expansion showed a consistent rate of 1-12.

But with "petal" bullets, including the Barnes X and Winchester Fail Safe (the two major petal-type bullets available at the time) the "wound channel" showed a progressive INCREASE from the barrel's twist-rate. In other words, the rate of bullet "turn" increased as it penetrated.

Some theorized this was due to the petals themselves, which generally formed a "higher angle" propeller shape than the more rounded and erratic "mushroom" of the lead-cored premiums. The theory was that the petals "grabbed" more tissue than the conventional rounded mushroom.

There was considerable debate about all of this on the Campfire back then.

Shooting 6mm bullets in a 1:10 .243 and a 1:8 6XC I was under the impression that the tighter twist enhanced expansion.

It would be very interesting, making such a test in a wax medium like John mentions.

John, aren't you tempted? :-)

Alvaro

Would it be safe to assume, with lead bullets, the higher expansion would result in a decrease in penetration?

99% of the time I roll my own ammo but a few years ago the LGS had a screamin deal on Black Hills 223 ammo with Hornady 60gr V-Max bullets,
I had never used this bullet before so I thought they were worth a try. I first tried them in my 1-12"twist CZ527 to see if they would stabilize with that slow of a twist, they shot quite well for factory ammo and I shot many prairie dogs, several rock chucks and a couple coyotes with them. I was quite impressed.

On a day that I was at the range chronographing I decided to try this ammo in one of my 1-8"twist AR's. The ammo shot exactly what they advertise out of my CZ at 3050fps a bit slower out of my AR at 2960fps.

It was time to try this ammo out on critters with my AR............WOW what a difference !!!!!!!!!! simply vaporizes prairie dogs, maximum air time on rock chucks, and makes a bloody mess with coyotes.

So at least with this bullet I would say there is a decrease in penetration, with my 1-12"twist CZ if you didn't hit bone it could penetrate through a coyote, with my 1-8"twist AR I have yet to ever see an exit hole.

Twist rate certainly can (usually does) have an impact on bullet expansion and, therefore, penetration.

It all relates to the RPM of the projectile. The RPM is a factor of muzzle velocity and rate of twist.

It is the reason downloading to simulate long range performance doesn't usually work very well.

It is also one of the problems the .40 S&W experiences. The SAAMI spec for .40 S&W is a 1:16 twist. The Glock (and many others since) has approximately a 1:10 twist rate.

I have two 20 practicals that I use on prairie dogs. One is a 1-11 twist and the other a 1-8 twist. Shooting the same bullet the 1-8 is noticeably more explosive. I can't even shoot the 39 gr sierra blitzking out of the 1-8 twist, it'll explode about half of them a few feet in front of the muzzle. I have to shoot the 40 gr vmax out of it.

Not at home to look, but didn't A-squares reloading manual have a section on bullet penetration with one of the factors being twist rate?

On AR there is a thread in the big bore section called Terminal Performance. Faster twist increased straight line penetration with solids

This is all the more reason to go with a faster twist. WDM Bell (Karamojo) theorized that the 220 Swift performed well on Red Stag beyond the range where the velocity was high due to the rotation of the bullet which slows much slower than velocity loss. So we have been dealing with this since the turn of the last century.

There have been test on both expansion and penetration that supports this. Using solids there might be an ideal velocity window for straight line penetration and it is not necessarily as fast as you can push them. A faster twist may increase this velocity/penetration window.

I spoke with him a few times before he passed along with what Bob and others running fast twisted big game rifles saw and I’m a believer myself. I can say it doesn’t hurt from what I see.

Bill convinced me and several others I know...

I spoke with him just prior to his passing as well. Was looking for some bullets and ended up on the phone with him for over an hour. Great guy.

Mule Deer,

With regards to your tests.

It stands to reason that if forward motion slows considerably faster
than twist, effective twist increases.


If a bullet is fired from a 1-12 rifle and loses 1/3 of its velocity, but no rotational speed,
It would then be turning once in every eight inches.

Yeah, him and others are the reason we set up my brothers 300 Win with the 8 twist Krieger and run about everything we own with pretty quick twists. Pulled a 200 AB from a decent bull shot at 675 yards this year, started at 2968 FPS. Bullet had real good expansion at that range, even though it was getting down in speed.

Pulled a 200 AB from a spike elk from a 300 RUM started at nearly 3200 and impacted at 560 yards. I remember thinking I wouldn’t wanna push them too much further. Put me down as a lover of fast twists. Eventually my 300 RUM will get an 8 as well. Oh and both elk were pretty similar in shot angles so bone impact wasn’t a real thing on the 300 Win. We did connect with the far side front leg bone from the RUM.

300 Win 675 yards





300 RUM 560 yards





Neither are bad but I’d take what the 300 Wins showing me all day long if it was a blindfolded test.

Kinda reminds me of clay pigeons. Shoot one hanging on a wire and they just break. Shoot one thrown from a machine and they explode

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

Lots of them do at lower impact velocities. In any case, you might google “Buford Boone terminal ballistics”...