M193 Ball performance is well documented from a 20-inch barrel using the FBI protocols. But…
Just what is the ballistic gelatin performance of M193, from an original specification 1-14 twist, 20-inch barrel?
Curious.
The same as it is from a 1-9 and 1-7 twist barrels. Twist rate does not impact the terminal ballistics of a projectile.
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. There is much greater variation associated with AOA issues and fleet yaw problems.
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 1/7 twist for 5.56 mm weapons and 1/8 twist for 7.62x51 mm 16" barrels.
Thank you for that comprehensive reply!
I read that the M16 originally had 1:14 twist but testing revealed that in sub-freezing temps, the bullet is not properly stabilized. So they tightened it up to 1:12 (for use with M193).
I think what’s difficult for me to wrap my head around is the idea that the terminal ballistics of a marginally stable projectile (1/14) is essentially the same as from better stabilized projectile (1/7) all other things being equal. It would seem likely (to me) that yaw would tend to commence earlier, creating a different wound pattern. I appreciate the assistance I received in this matter.
The drag in tissue is dramatically higher than in air, so the relative small contribution of twist rate to stability does not alter the yaw depth very much. AOA on impact, bullet shape, and bullet CG are much more critical to yaw depth in tissue.
Got it. Makes sense. Thank you!