1:12 twist was the old A1 barrel, wasn't it Shakken? Here's an excerp taken out of the ar15.com ammunition FAQ.
"Why don’t all US military units carry M855?
The original ammo for the M16 was M193, with a 55gr copper-jacketed lead-core bullet. The rifling twist on the first M16s was 1 turn in 14 inches, or 1:14. This twist was selected simply because it was the twist rate commonly used in the .222 Remington-chambered varmint rifles that the .223 round was based on. During tests of the M16 in arctic regions, it was found that the slow 1:14 twist wasn’t fast enough to stabilize the bullet in the denser air. To correct this problem, the twist was tightened to 1:12, and all future M16s and M16A1s came with 1:12 barrels.
The M855 round, and particularly the M856 tracer round, are very long bullets, and require a faster twist rate in order to be stabilized in air. (Stabilization is a factor of caliber, velocity, and bullet length, not weight.) Firing M855 from a 1:12-twist rifle would result in an understabilized bullet that would only fly straight for about 90 yards, then veer off as much as 30° in a random direction. In order to prevent soldiers from accidentally firing M855 in 1:12-twist rifles, M855/SS-109 was given a green tip in countries that had previously issued 1:12-twist rifles and M193-spec ammo. This allows M855/SS-109 to be differentiated from plain-tipped M193. M16A2s, A3s, A4s, M4s, and M4A1s all have a 1:7 twist, and can stabilize both M855 and M193.
Military technical manuals warn against using M855 in M16A1s (or any 1:12 twist rifle) except in a “combat emergency,” and warn that it is not effective in such rifles beyond 95 yards.
See also:
http://www.povn.com/~4n6/index.htm"
"Why don’t all US military units carry M855?
The original ammo for the M16 was M193, with a 55gr copper-jacketed lead-core bullet. The rifling twist on the first M16s was 1 turn in 14 inches, or 1:14. This twist was selected simply because it was the twist rate commonly used in the .222 Remington-chambered varmint rifles that the .223 round was based on. During tests of the M16 in arctic regions, it was found that the slow 1:14 twist wasn’t fast enough to stabilize the bullet in the denser air. To correct this problem, the twist was tightened to 1:12, and all future M16s and M16A1s came with 1:12 barrels.
The M855 round, and particularly the M856 tracer round, are very long bullets, and require a faster twist rate in order to be stabilized in air. (Stabilization is a factor of caliber, velocity, and bullet length, not weight.) Firing M855 from a 1:12-twist rifle would result in an understabilized bullet that would only fly straight for about 90 yards, then veer off as much as 30° in a random direction. In order to prevent soldiers from accidentally firing M855 in 1:12-twist rifles, M855/SS-109 was given a green tip in countries that had previously issued 1:12-twist rifles and M193-spec ammo. This allows M855/SS-109 to be differentiated from plain-tipped M193. M16A2s, A3s, A4s, M4s, and M4A1s all have a 1:7 twist, and can stabilize both M855 and M193.
Military technical manuals warn against using M855 in M16A1s (or any 1:12 twist rifle) except in a “combat emergency,” and warn that it is not effective in such rifles beyond 95 yards.
See also:
http://www.povn.com/~4n6/index.htm"
