Modes Of Fire

The M16A3 and M4A1 rifles function in either the semiautomatic or automatic mode. The M16A2, M16A4, and M4 carbine function in either the semiautomatic or three-round burst mode.

a. Semiautomatic Fire Mode (M16-/M4-series). The disconnector is a mechanism installed so the firer can fire single rounds. It is attached to the trigger and rotated forward by action of the disconnector spring. When the recoil of the bolt carrier cocks the hammer, the disconnector engages the lower hook of the hammer and holds it until the trigger is released. Then the disconnector rotates to the rear and down, disengaging the hammer and allowing it to rotate forward until caught by the nose of the trigger. This prevents the hammer from following the bolt carrier forward and causing multiple firing. The trigger must be squeezed again before the next round will fire.

b. Automatic Fire Mode (M16A3 Rifle, M4A1 Carbine Only). When the selector lever (Figure 4-11) is set on the AUTO position, the rifle continues to fire as long as the trigger is held back and ammunition is in the magazine. The functioning of certain parts of the rifle changes when firing automatically.

(1) Once the trigger is squeezed and the round is fired, the bolt carrier group moves to the rear and the hammer is cocked. The center cam of the selector depresses the rear of the disconnector and prevents the nose of the disconnector from engaging the lower hammer hook. The bottom part of the automatic sear catches the upper hammer hook and holds it until the bolt carrier group moves forward. The bottom part strikes the top of the sear and releases the hammer, causing the rifle to fire automatically.

(2) If the trigger is released, the hammer moves forward and is caught by the nose of the trigger. This ends the automatic cycle of fire until the trigger is squeezed again.

M16a2 Burst Cam
Figure 4-11. Automatic fire mode.

c. Burst Fire Mode (M16A2/A4 Rifle, M4 Carbine). When the selector lever is set on the BURST position (Figure 4-12), the rifle fires a three-round burst if the trigger is held to the rear during the complete cycle. The weapon continues to fire three-round bursts with each separate trigger pull as long as ammunition is in the magazine. Releasing the trigger or exhausting ammunition at any point in the three-round cycle interrupts fire, producing one or two shots. Reapplying the trigger only completes the interrupted cycle; it does not begin a new one. This is not a malfunction. The M16A2/4 and M4 disconnectors have a three-cam mechanism that continuously rotates with each firing cycle. Based on the position of the disconnector cam, the first trigger pull (after initial selection of the BURST position) can produce one, two, or three firing cycles before the trigger must be pulled again. The burst cam rotates until it reaches the stop notch.

M16a2 Exploded View
Figure 4-12. Burst fire mode.

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