The M855A1, a 5.56 mm Ball ammunition, is an enhanced performance round for today’s combat and training environments. It works with the M4 Carbine, the M249 machine gun, and the H&K and M16A2 rifles. It is suitable for use in most weapons with a 1-in-7 barrel twist.
The M855A1 has a copper-jacketed steel core, differing from the previous M855 round which has a lead core. The so-called “green ammo” not only has increased penetration of armor and hard targets, but allows the Army to be more environmentally friendly on its ranges and training environments.
The M855A1 is optimized for the shorter-barrel M4 where the standard M855 was optimized for the M16. The powder burns faster and creates more pressure, and has the effect of reducing flash. The round yaws like the M855, but its yaw is more consistent and predictable, meaning its terminal effects are not yaw dependant.
The Army plans to replace its entire inventory of M855 rounds with the M855A1 round.