Purposeful Tumble in an AR Pistol

[War Squirrel]

I was wondering if it was possible to recreate the "meat axe" effect from Vietnam in a close range weapon like an AR pistol.

The 1-14 twist rate in the original M16, coupled with 55gr bullets, caused the rounds to wobble on their rotational axis in flight. This caused unacceptable accuracy for the rifle, although it had tremendously lethal terminal ballistics due to the instability of the bullet.

So, if we use a weapon such as an AR pistol that isn't meant for the kinds of ranges an infantry rifle would be (and thus doesn't really need pinpoint accuracy), give it the slow 1-14 twist barrel, and light 55gr rounds, could we recreate those terminal ballistics in a weapon where they would be not be detrimental, but desirable? Would too much velocity be lost to the short barrel?

 

[HorseSoldier]

Or the enhanced lethality was actually caused by bullet fragmentation, which relies on velocity and which an AR pistol isn't going to do you any favors with -- the velocity loss is probably a show stopper.

 

[War Squirrel]

Would you need less velocity to cause fragmentation if the bullet wasn't stable?

 

[rcmodel]

caused the rounds to wobble on their rotational axis in flight. This caused unacceptable accuracy for the rifle,

That isn't true.

The bullet was stable, and acceptably accurate at typical ranges at typical air temperatures.

The problem came in cold dense artic air at extended range of 400+ yards when the bullet slowed down and became unstable.

As an aside, the .222 Rem and .222 Rem Magnum the .223 was based on has always used 1/14 twist barrels and often still do.

They will stabilize bullets up to 63 grain semi-pointed.
And they are among the most accurate varmint calibers ever made.

In fact the .222 Rem held the world benchrest record for many years.

rc

 

[Sam Cade]

Q. Didn't tightening the twist rate from 1:14 to 1:12 reduce the wounding potential of M193?

No...

...though unfortunately this is widely believed. When the M16 was first used in Vietnam, it was assumed that the smaller 5.56mm round would make much smaller wounds than the 7.62mm M80 round fired from the M14. Everyone was surprised to learn that M16 wounds were often much more severe. In order to explain this discrepancy, it was theorized that the slow 1:14 barrel twist made the bullet less stable in flesh and caused it to tumble, resulting in the large wounds. In fact, the slow twist only made the bullet less stable in air. Any pointed, lead core bullet has the center of gravity aft of the center of the projectile and will, after a certain distance of penetration, rotate (yaw) 180° and continue base-first. This is where the appearance of "tumbling" came from.

The actual cause of the larger-than-expected wounds was not a result of this yawing of the bullet, but of the velocity of the bullet coupled with the bullet's construction. M193 bullets have a groove or knurl around the middle, called a cannelure. This allows the mouth of the case to be crimped on to the bullet, preventing the bullet from being pushed back into the case during handling and feeding. The cannelure also weakens the integrity of the bullet jacket.

When the bullet struck flesh at a high-enough velocity, the bullet's thin jacket, weakened by the cannelure, could not survive the pressure of moving sideways through the dense flesh. Instead, the bullet would only rotate about 90°, at which point the stresses were too much for the bullet jacket and the bullet would fragment. The results were a wound that was far out of proportion to the size of the bullet. Yet, the twist rate of the barrel and therefore the rotation speed of the bullet, is not a factor in the fragmenting equation.

M855 ammo works exactly the same way, though due to its heavier bullet, it has less muzzle velocity. Less muzzle velocity translates to a shorter range in which the bullet retains enough velocity to fragment, compared to M193.

http://ammo.ar15.com/ammo/project/term_tighttwist.html

 

[silicosys4]

Here's a good article concerning twist rates and fragmenting ammo.

My understanding is, with standard 55 grain and 62 grain rounds, velocity is key to fragmentation, and therefore performance. I read an article once, wish I could properly cite it...that broke down the performance of those two standard military loadings by range. Basic conclusion was, a 20" barrel 5.56x45 was only effective out to about 200 yards, after that it was just punching .22 caliber holes in people. The reason was, velocity fell off too much after 200 yards to offer ANY fragmenting, and the really "explosive" fragmenting only happened out to about 100 yards, or about 2700 fps or so. Under 100 yards, "explosive" fragmenting caused a very large permanent wound cavity, not yawing or tumbling, with the bullet essentially separating into one large fragment (the nose of the bullet) and many small fragments that used to be the base...this happens because the bullet starts to tumble fairly early, with high velocity still...but as soon as it rotates off axis, the nose forward of the canellure is "bent" at a 90 degree angle to the rest of the bullet and separates, remaining in one piece, while the base of the bullet fragments, not being held together fully enclosed by the copper jacket like the nose section. This "explosive" fragmentation only happens to 100 yards; after 100 yards the fragmentation becomes much less severe, until at 200 yards the bullet basically stays together, only bends a little at the cannelure, and only a few fragments of lead are squeezed out the base.

http://forums.second-amendment.org/index.php?topic=3708.0;wap2

Now, a 16" carbine will only retain that "explosive" fragmentation energy out to about 80 yards, because of the velocity drop from the shorter barrel.
I'd be surprised if a pistol ar-15 would have the velocity to fragment, even at the muzzle.

Here is a really good article describing exactly what I was talking about above
http://ammo.ar15.com/ammo/project/term_velocity.html

 

[War Squirrel]

Ah! Well dang, there goes that idea. Thanks guys!

 

[rcmodel]

I was wondering if it was possible to recreate the "meat axe" effect from Vietnam in a close range weapon like an AR pistol.

Yes it is.

Nobody says you have to shoot FMJ military ammo to do it though.

Just load lighter 50-55 grain ballistic-tip bullets like Hornady V-Max or Nosler BT's.
Same as Hornady Urban TAP ammo.

http://www.hornadyle.com/products/rifle-ammunition/223-remington/55-gr-tap-urban

The explosive effect out of even a short AR pistol would be quite devestating.

rc

 

[adelbridge]

I have shot hogs with IMI 62gr M855 FMJ with 1:9 and I get a little entrance and 3"-4" diameter exit wound. I am fairly certain the bullet destabilizes when they hit soft tissue and tumble through flesh. The internal damage is sometimes greater than 4" diameter wound channel and pretty much always a soupy mush and I think the tough hog hide resists tearing on exit.

 

[LoonWulf]

I agree with RC. Use a mid weight, fast expansion bullet, and your terminal effects well be impressive to say the least.
I used 55grn ballistic tips for a great deal of my deer control when we owned our farm (and before anyone complains about it I never lost an animal), penetration was usually about 4-8"s and the destruction of soft tissue was very impressive. I load my mini-14 with 60grn Vmax and use it for goats, as well as home defense.

 

[DPris]

As RC said, the "wobble" wasn't in flight. It was the tumbling effect inside the target on impact that could create massive wounds.

Just be aware that the short barrels on AR pistols cause a major velocity loss that will affect terminal performance.
Denis