Commanders and marksmanship trainers must understand some aspects of ballistics to teach the principles of zeroing and engagement of long-range targets. Ballistics is a science dealing with the motion and flight characteristics of projectiles. The study of ballistics in rifles is divided into three categories: internal, external, and terminal.
• Internal ballistics concerns what happens to the bullet before it leaves the muzzle of the rifle.
• External ballistics deals with factors affecting the flight path of the bullet between the muzzle of the rifle and the target.
• Terminal ballistics deals with what happens to the bullet when it comes in contact with the target.
a. Internal Ballistics. The overall dimensions of the combat service 5.56-mm cartridges are the same, which allows cartridges to be fired safely in M16A1 or M16A2 rifles and the M4 carbine. There are internal differences that affect firing accuracy. An ammunition comparison is provided in Figure 5-32.
(1) The increase in projectile length, weight, and configuration of the M855 bullet requires different twists in the barrels, lands, and grooves to stabilize the bullet in flight. The M16A1 has a 1:12 barrel twist (the bullet rotates once for every 12 inches of travel down the barrel). The M16A2/A3/A4 and the M4 carbine has a 1:7 barrel twist (the bullet rotates once for every 7 inches of travel down the barrel).
(2) The M16A1, with its 1:12 twist, does not put enough spin on the heavier M855 bullet to stabilize it in flight, causing erratic performance and inaccuracy for training or full combat usage (30.48- to 35.56-centimeter shot group at 91.4 meters and 72-inch shot group at 274.2 meters) (Figure 5-33). Although firing the M855 cartridge in the M16A1 rifle is safe, it should only be used in a combat emergency, and then only for close ranges of 91.4 meters or less.
M855 (100 Yards) M855 (300 Yards)
_M855 BALL_Ml93 BALL_
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