Q. Can I fire M855/SS-109 in my 1:12 twist barrel?
...it won't be stabilized properly and after 90-95 yards, it will typically veer off in a random direction. You often won't hit paper at 100 yards. Though it won't hurt your rifle to fire this ammo, it is not recommended. Military manuals warn that it should only be fired in 1:12 twist barrels in a "combat emergency."
Fact: The Scoop from the Army's Ammunition Information Notice (61-01) "INTERCHANGEABILITY OF 5.56MM BALL, TRACER AND BLANK AMMUNITION."
"Cartridges M855 and M856 ammunition are extremely inaccurate when fired in the M16 and M16A1. The M16 and M16A1, with their 1:12 twist, do not impart enough spin on the heavier M855/M856 projectile to stabilize it in flight causing erratic performance and resulting inaccuracy.
Therefore, while safe to fire in M16 and M16A1 they should only be used in an combat emergency and then only for close ranges 91.4 meters (100 yards) or less."