Internal Ballistics Factors

The overall dimensions of the combat service 5.56-mm cartridges are the same, which allow cartridges to be fired safely in M16A1 or M16A2 rifles. There are internal differences that affect firing accuracy. An ammunition comparison is provided in Figure F-14.

This increase in projectile length, weight, and configuration requires different twists in the barrels, lands, and grooves to stabilize the M855 bullet in flight (Figure F-15). The M16A1 has a 1:12 barrel twist. (The bullet rotates by the lands once for

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