Barrel Twist in relation to Bullet Yaw

[Heavy Metal]

So if the 1-14 twist rate was considered "Ideal" by Stoner for the 55gr bullet could the same be said for the 1-12 twist rate some claim is ideal for the 62gr version. Does anyone have a link to testing which proves or disproves this assumption?

A 1/12 twist will not even stabilize the long 62gr SS109 projectile found in M855 ammo.

This renders it far from ideal.

[DocGKR]

Please...

The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists. Similar testing was conducted with 5.56 mm 62 gr M855 FMJ ammunition fired in 1/9 and 1/7 twist barrels. Again, no difference in terminal performance was noted. There are some projectiles where the terminal performance can be effected by twist rate, but these are not generally in military use. Also, if the bullet is not adequately stabilized in flight, then alterations in the wound profile will be evident.

Twist rate can definitely effect external ballistics. For example, in testing ammunition at the CHP Academy in the mid 1990’s, a number of lightweight, thin-jacketed, relatively high velocity .223 varmint loads were observed to disintegrate in mid-air a few yards from the muzzle when fired from fast 1/7 twist weapons, but not in slower twists; the Federal 40 gr Blitz loading was particularly problematic in this regard. Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.

Personally, I prefer 1/8 or 1/7 twist for 5.56 mm weapons.

[rubber ducky]

Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.

I was going to add... with my limited knowledge regarding twist rates and bullet stabilization - my understanding is that its really not the bullet weight, but rather bullet length... So, if you shoot same weight bullets of different lengths, you will get different results - assuming you're using the same barrel.

My apologies if this contributed nothing to this thread.

Sincerely,

rd

[K.L. Davis]

Yep, it is really the the length of the projectile...

1:8 twist will spinup anything that you can fit in a magazine and works for nearly all conditions and ranges a person could ever encounter -- benchrest shooting is a world where a person can learn and obsess about all of the minutia... but rest assured that 99.9% of the time, 1:8 RoT (or there abouts) is taking care of everything for you, no user input needed

[rubber ducky]

... 99.9% of the time, 1:8 RoT (or there abouts) is taking care of everything for you, no user input needed

if I recall correctly, I think the "magic number" was something along the lines of 1:7.8

[dewatters]

As originally designed by Eugene Stoner, the M16 had a 1/14" twist. This kept the 55gr bullet at the ragged edge of stability. When the bullet impacted flesh it rapidly yawed and fragmented. In military testing by the Air Force in arctic conditions, it was determined that accuracy became unacceptable at 65 below and a change to 1/12 twist was recommended to further stabilize the round for arctic conditions - just in time for the jungles of Vietnam. "Misfire", pg 490.

The USAF report that led to the adoption of the 1:12" twist, "Exterior Ballistics of the AR-15 Rifle", is available online at DTIC. One thing to note is that the research was performed under the supervision of Gerald A. Gustafson. One of the early SCHV advocates at Aberdeen, Gustafson transferred to the USAF Armament Center in the mid-'50s after Dr. Carten refused to fund further SCHV cartridge development by Aberdeen's Small Arms and Aircraft Weapons Branch.*

As someone else mentioned 1:14" twist was commonplace for other rifle cartridges using 0.223-0.224" projectiles. However, these cartridges were typically loaded with lighter and shorter projectiles than the 55gr FMJ-BT used in the .223 Remington. The comment about -65F is a smokescreen. While they did test to temperatures that low, stability of the 55gr FMJ-BT projectile from 1:14" twist barrels had already gone to pot by 0F. They indicated that the 1:14" twist would be adequate for a flat-base 55gr FMJ, but this would reduce the maximum range of the projectile.**

As the USAF was the primary user of the AR-15/M16 in 1963, their needs were considered paramount. The USAF has bases in Alaska and other nasty cold places. Major deployments to South Vietnam of US Army combat units with the XM16E1 did not occur until 1965. In the meantime, McNamara refused to allow additional AR-15/M16 to be issued to the South Vietnamese after the ARPA trials. Mass issue of the M16A1 to regular South Vietnamese military units did not begin until after McNamara left office in 1968.

When the rifling twist controversy reared its head again in 1967, a large batch of 1:14" barrels (~1,000) were made and assembled into rifles for comparison testing against an equal number of rifles with 1:12" barrels. In the summer of 1967, Colt testing found that the 1:14" barrels shot groups twice as large as the 1:12" barrels. Colt lobbied against the reversion in rifling twist as before the transition to 1:12" twist barrels, they had to reject 10% of the 1:14" twist barrels in acceptance testing due to insufficient accuracy.

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*On a side note, the USAF's Dale M. Davis was assigned to Aberdeen and worked under Gustafson during the SCHV experiments. Davis returned to the USAF Armament Center around the same time Gustafson transferred. Davis was later responsible for the creation of the "Arm Pistol" concept which led to the Colt IMP.

**Ironically, Remington's 55gr FMJ-BT design was shorter than the original Stoner/Sierra projectile design. The BRL at Aberdeen later found that if they wanted to revert to the Stoner/Sierra projectile, the proper rifling twist would have been around 1:10". As they had already replaced the 1:14" barrels, the military wasn't going to turn around and then scrap all of the 1:12" barrels.

[benthughes]

The formula formula to determine the gyroscopic stability of a bullet is given by:



So an increase in w by going to a faster twist rate in the barrel is more than offset by the thousandfold increase in p once the bullet encounters tissue. In other words: Tissue is so much more dense than air that the slightly faster rate of bullet rotation is negligible; the bullet will be completely unstable as soon as it encounters tissue.

Ummmm....Duh!

[DocGKR]

There is ongoing new research indicating that with some expanding projectiles, especially in shorter barrels, FASTER twists produce improved terminal ballistic performance...

[Ernst]

Ummmm....Duh!

Exactly, I mean, even a pre-schooler knows this stuff, right?

[vicious_cb]

There is ongoing new research indicating that with some expanding projectiles, especially in shorter barrels, FASTER twists produce improved terminal ballistic performance...

Not sure how I missed this gem of information. Any updates on this phenomenon? Any theory on why this occurs?