Can someone explain the differance in the bullets and barrel twists in an AR-15. [Archive]

sfc ret

04-12-2002, 03:31 PM

M16A1 has the 1/12 twist....for every 12 inches the bullet does one rotation. the m16A2 has a 1/7 twist for every 7 inches the bullet does one rotation.....the accuracy increases dramatically since the spinning of the bullet stabilizes even more in flight, and the tracjectory is more straight....most of the commercial ARs have a 1/9 twist...a happy medium between the 2, so you can fire both rounds....although you can also fire 55 grain out of a 1/7.

if you fire the 62 gr. round out of a 1/12 twist the round will tumble. had a soldier with a A1 and fired on the zero range.....it keyholed the target.

Circuits

04-12-2002, 03:53 PM

The longer a bullet is, the faster the twist required to stabilize it. It's really not related to bullet weight, other than a solid lead bullet in a .224 bore has to get longer as it gets heavier. A boat-tailed pointed bullet design of the same weight as a flat-base flat-tip design is longer, and will require a slightly faster twist. "overstabilization" is usually not detrimental, except if the jacket material is too weak for the faster rotation, the bullet can bulge or disintegrate in flight. This has been observed with very light 40gr varmint type rounds being fired from 1x7 twist barrels.

The original AR-15/M16 had a 1x14 barrel common to .222 sporting rifles of the day (mostly firing 40-50gr bullets). 1x14 is not fast enough to properly stabilize the 55gr M193 military ball round in very cold air, as the Army found out during arctic trials. This is also probably the original source of the 'tumbling bullet' stories about the M16's round. The Army switched to 1x12 when they adopted the M16, and all subsequent M16 and M16A1's have had 1x12 barrels.

1x12 is more than satisfactory for stabilizing the 55gr M193 round.

When the Army adopted the M249 Squad Automatic Weapon, they changed from M193 to the longer-range, heavier M855 cartridge, based on the Belgian SS109 bullet designed specifically for the SAW. To simplify logistics, the Army also adopted the M855 round for the M16A2. M855 needs a 1x9 twist to stabilize properly, due to its much greater bullet length. The M856 tracer round used in conjunction with the M855 ball round is even longer still, and really needs a twist of about 1x5. The Army ended up adopting 1x7 as the twist for the M16A2 barrel, as a compromise between the 1x9 and 1x5. Using the 1x5 twist would result in faster barrel wear, and your average soldier doesn't fire tracer very much.

Commercial rifle production mostly uses the 1x9 twist which is optimum for M855 ammo, and still good for M193 type 55gr ammo.

Competition shooters can get AR barrels in 1x10 or 1x8, as well, to get the optimum twist for a particular bullet they fire in competition. 1x8, especially, is used by long range shooters for heavy long-distance match rounds up to 85gr.