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

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