The machine gun is designed to function automatically as long as ammunition is fed into the gun and the trigger is held to the rear. Each time around is fired, the parts of the machine gun function in a certain sequence. Many of the actions occur simultaneously and are only separated for teaching purposes. The sequence of operation is known as the cycle of operation.
For ease of understanding, the complete cycle of operation is discussed in the following eight steps:
1. Feeding: Around is positioned in the feed tray groove.
2. Cambering: Around is stripped from the belt and placed in the chamber.
3. Locking: The bolt is locked inside the barrel socket.
4. Firing: The firing pin strikes and initiates the primer of the cartridge.
5. Unlocking: The bolt is unlocked from the barrel socket.
6. Extracting: The empty case is pulled from the chamber.
7. Ejecting: The empty cartridge case is thrown from the receiver.
8. Cocking: The sear engages the sear notch.
The cycle starts by putting a round in the feed tray groove and then pulling the trigger, releasing the sear from the sear notch (fig. 3-70). It stops when the trigger is released and the sear again engages the sear notch in the operating rod. When the trigger is held to the rear, the rear of the sear is lowered and disengaged from the sear notch. This allows the operating rod and bolt to be driven forward by the expansion of the operating rod spring. Now that the gun is functioning, the steps of the cycle can be traced
As the bolt begins its forward movement, the feed cam is forced to the right, causing the feed cam lever to pivot in the opposite direction and forcing the feed pawl over the next round in the belt, ready to place it in the feed tray groove when the rearward action occurs again. As the bolt moves to the rear after the firing, the cam roller in the top of the bolt forces the feed cam to the left. The feed cam lever is forced to pivot, moving the feed pawl to the right, placing a round in the feed tray groove, as shown in figure 3-71.
As the bolt travels forward, the upper locking lug engages the rim of the cartridge. The pressure of the front and rear cartridge guides hold the round so that positive contact is made with the upper locking lug of the bolt. The front cartridge guide prevents the forward motion of the link as the round is stripped from the belt. The upper locking lug carries the round forward, and the cambering ramp causes the nose of the cartridge to be caromed downward into the chamber, as shown in figure 3-72. When the round is fully seated in the chamber, the extractor snaps over the rim of the cartridge, and the ejector on the face of the bolt is depressed.
Figure 3-70.-Sear disengaging from sear notch.
Figure 3-73.-Weapon locked, ready to fire.
As the round is chambered, the bolt enters the barrel socket. The upper and lower locking lugs contact the bolt caroming surfaces inside the barrel socket and start the rotation of the bolt clockwise. The action of the operating rod yoke against the bolt caroming slot, as the operating rod continues forward, causes the bolt to complete its one-quarter turn clockwise rotation (fig. 3-73). Locking is then completed.
After the bolt reaches its fully forward and locked position, the operating rod continues to go forward, independently of the bolt, for a short distance. The yoke, engaged between the firing pin spools, carries the firing pin forward. The striker of the firing pin protrudes through the aperture in the face of the bolt, strikes the primer of the cartridge, and ignites it. This action is shown in figure 3-74.
Figure 3-75.-Unlocking action of gases.
After the cartridge is ignited and the projectile passes the gas port, part of the expanding gases enter the gas cylinder through the gas port. The rapidly expanding gases enter the hollow gas piston, as shown in figure 3-75, and force the piston to the rear. The operating rod, being in contact with the piston, is also pushed to the rear. As the operating rod continues to the rear, the operating rod yoke acts against the bolt caroming slot to cause the bolt to begin its counterclockwise rotation. The upper and lower locking lugs of the bolt, contacting the bolt caroming surfaces inside the barrel socket, cause the bolt to complete its one-quarter turn rotation (counterclockwise) and unlock the bolt from the barrel socket. Unlocking begins as the yoke of the operating rod contacts the curve of the bolt caroming slot and ends as the bolt clears the end of the barrel socket.
While unlocking is going on, extraction is beginning. The rotation of the bolt, in unlocking, loosens the cartridge case in the chamber. As the operating rod and bolt continue to the rear, the extractor (gripping the rim of the cartridge) pulls the cartridge case from the chamber. As the case is withdrawn from the chamber, the ejector spring expands. The ejector presses on the base of the cartridge case, forcing the front of the spent case against the right side of the receiver, as shown in figure 3-76. As the bolt continues to the rear, the action of the ejector pushing against the base of the cartridge case and the extractor gripping the right side of the case cause the cartridge case to spin from the gun as the case reaches the ejection port. The empty link is forced out of the link ejection port as the rearward movement of the bolt causes the next round to be positioned in the feed plate groove.
As the expanding gases force the gas piston to the rear, the operating rod is initially moved independently of the bolt. The yoke of the operating rod acts against the rear firing pin spool, withdrawing the firing pin from the primer of the spent cartridge case. The action of the operating rod yoke continuing to the rear against the rear firing pin spool fully compresses the firing pin spring. As long as the trigger is held to the rear, the weapon will continue to complete the first seven steps of functioning automatically y. When the trigger is released and the sear again engages the sear notch, the cycle of functioning is stopped and the weapon is cocked.