OPERATION AND FUNCTION
This chapter discusses the operation of the M249 automatic rifle. This includes loading, unloading, cycle of functioning, adjusting the sight, and using the bipod.
Figure 3-1. Loading.
NOTE: The 20- or 30-round magazine is for emergency use only when linked ammunition is not available.
Figure 3-3. Loading a magazine.
Before you raise the feed cover, move the weapon away from your face
so that you are not exposed to the open chamber.
b. Magazine-Fed. Push the magazine release tab down and pull the magazine from the magazine well. Raise the feed cover and perform the five-point safety check.
b. The sequence of functioning is as follows.
(2) Chambering. As the bolt travels forward, the upper locking lug engages the rim of the round. The pressure of the front and rear cartridge guides holds the round so that positive contact is made with the upper locking lug of the bolt. The front cartridge guide prevents forward movement of the link as the round is stripped from the belt. The upper locking lug carries the round forward. The chambering ramp causes the nose of the round to be cammed downward into the chamber. When the round is fully seated in the chamber, the extractor snaps over the rim of the round, and the ejector on the rail inside the receiver is depressed.
(3) Locking. As the round is chambered, the bolt enters the barrel socket. The upper and lower locking lugs contact the bolt camming surfaces inside the barrel and start turning the bolt clockwise. The action of the bolt into the slide assembly, as the piston continues forward, turns the bolt to complete its 90-degree (one-quarter turn) clockwise rotation. Locking is now complete.
(4) Firing. After the bolt is fully forward and locked, the piston continues to go forward, independent of the bolt, for a short distance. The slide assembly carries the firing pin through the face of the bolt. The firing pin strikes the primer of the round and the primer fires the round.
(5) Unlocking. After the round is fired and the bullet passes the gas port, part of the expanding gases go into the gas regulator through the gas plug. The rapidly expanding gases enter into the gas cylinder from the gas regulator, forcing the piston to the rear. As the piston continues to the rear, the slide assembly, also moving to the rear, causes the bolt to begin its counterclockwise rotation. The upper and lower locking lugs of the bolt contact the bolt camming surfaces inside the barrel socket and, as the bolt continues toward the rear, it completes a one-quarter turn counterclockwise. The rotation and movement to the rear unlocks the bolt from the barrel socket.
(6) Extracting. Extracting begins during the unlocking cycle. The rotation of the bolt loosens the cartridge case in the chamber. As the piston and bolt move to the rear, the extractor pulls the cartridge case from the chamber.
(7) Ejecting. As the cartridge case is pulled from the chamber, the bolt passes by the ejector. This causes the ejector clip to expand, forcing the ejector to push the expended cartridge. The extractor grips the right side of the cartridge and causes it to spin from the weapon as it reaches the ejection port. The empty belt links are forced out the link ejection port as the rearward movement of the bolt causes the next round to be positioned in the tray groove.
(8) Cocking. The piston assembly acts against the firing pin, pulling the firing pin from the primer of the spent cartridge case. The action of the piston assembly, continuing to the rear with the firing pin, releases the compression of the firing pin spring. As long as the trigger is held to the rear, the M249 AR will continue to complete the eight steps of functioning automatically. When the trigger is released and the sear again engages the sear notch, the cycle of functioning is stopped and the weapon is cocked. To prevent undue wear to the sear and sear notch, the automatic rifleman must hold the trigger firmly to the rear during firing.
b. Windage. Adjustments for windage are made by traversing the rear sight right and left along the sliding scale. The sliding scale is marked or graduated with index lines. Each index line is equal to 1/2-mil change in direction or 1/2-cm change of impact at 10 meters. Rotation of the windage knob (closest to the muzzle end) toward the muzzle (counterclockwise) moves the rear sight aperture right, which moves the strike of the rounds right; while rotation toward the buttstock (clockwise) moves the aperture left, which moves the strike of the rounds left.
c. Corrections. One click of the windage knob or peep sight moves the strike of the round 1/2 cm at a range of 10 meters or 1/2 meter at 1,000 meters. To make corrections, Table 3-1 is used as a reference.
Figure 3-4. Sliding scale on sight.
Table 3-1. Windage and elevation (peep sight) correction chart.
Figure 3-5. Lowering the bipod.
c. Fold the bipod legs when transporting the weapon. Hold the two legs together, pull back under the handguard, and release so that the hooks on the legs grip the handguard. The bipod can be folded only when the legs are in the closed position (Figure 3-6).
Figure 3-6. Folding of the bipod under the handguard.