Maintenance and operation of the Mk 87 Mod 1 line-throwing rifle adapter kit is covered in NAVSEA SW350-A1-MMO-010. Kit maintenance is also covered by a 3-M Systems MRC.

MACHINE GUNS

LEARNING OBJECTIVE Discuss the operation and maintenance of the machine guns currently used by naval forces afloat.

Table 3-2.-Range Data

The Navy currently uses four machine guns-the M2 .50 caliber Browning, the 7.62-mm M-60, the 20-mm Mk 16 Mod 5, and the 25-mm M242 chain gun. In this section we will provide you with some operational and maintenance information on these four weapons. Discussion on the Mk 16 Mod 5 machine gun will be brief since it is in the process of being phased out and replaced by the 25-mm M242.

THE .50-CALIBER BROWNING MACHINE GUN

Browning machine guns (abbreviated BMGs) are standard Army weapons used by the Navy. The .50-cal. BMG now used by the Navy and Army is the M2. The M2 BMG is only equipped with an air-cooled heavy barrel (HB) since the light air-cooled barrel is no longer in use.

For a time the .50-cal. BMG (fig. 3-64) was not used aboard surface ships but has since been installed on most types of ships and landing craft.

Because of its complexity, the mechanisms and the principles of operation of the .50-cal. BMG will not be taken up in this section of the chapter. For detailed information on the .50-cal. BMG, refer to the Army's FM 23-65.

The main characteristics of the .50-cal. BMG (M2) are as follows:

General Description

The .50-cal. HB M2 Browning machine gun is a belt-fed, recoil-operated, air-cooled machine gun. The gun is capable of semiautomatic as well as automatic fire.

The gun is capable of alternate feed (ammunition can be fed from either the right or left side of the receiver) by repositioning some of the component parts; however, under most circumstances, the gun is fed from the left side. A disintegrating metallic link belt is used in feeding. In preparation for firing, the first round requires manual operation.

The force for recoil operation of the weapon is furnished by the expanding gases and is controlled by various springs, cams, and levers.

Air cooling of the weapon is permitted through maximum exposure to the air of the barrel and receiver. Perforations in the barrel support allow air to circulate around the breech end of the barrel and help to cool the parts. The heavy barrel is used to retard early overheating.

Operating the .50-Cal. BMG

The safest and best way to operate a .50-cal. machine gun is to follow the correct procedures. By following set procedures, you prevent damage to the gun and possible injury to you or others in the area. The operating procedures of the .50-cal. BMG include prefire, checks, loading and unloading, and postfire checks.

The primary prefire check requirement is the inspection of the weapons headspace and timing. This is done with a headspace and timing gauge (fig. 3-65). Headspace is the distance between the face of the bolt and the base of a cartridge case fully seated in the chamber. The timing of the gun makes sure that firing takes place with the bolt in the correct position (just before it reaches the full FORWARD position).

Figure 3-64.-.50-cal. HB Browning machine gun (M2). 3-34

Figure 3-65.-Headspace and timing gauge.

The operator must also select automatic or semiautomatic firing; automatic is the normal mode of operation. The mode of operation is determined by the position of the bolt-latch-release lock (fig. 3-66). For automatic firing, the bolt-latch release must be locked in the DEPRESSED position by the bolt-latch-release lock. To engage the bolt-latch-release lock, first depress the bolt-latch release. Then turn the bolt-latch-release lock counterclockwise until it hooks and retains the bolt-latch release in the DEPRESSED position.

Figure 3-66.-Bolt-latch release free of the bolt-latch-release lock.

On the command HALF-LOAD, with the cover closes, the double-loop end of the ammunition belt is inserted in the feedway until the first round is held by the belt-holding pawl (fig. 3-67). The retracting slide handle is then pulled all the way to the rear and released. With the bolt-latch-release lock positioned to engage the bolt-latch release, the bolt and retracting slide handle will move forward under pressure of the driving spring group, thus half-loading the gun. However, if the bolt-latch release is up and free of the bolt-latch-release lock, the bolt latch will hold the bolt to the rear. Push the retracting slide handle all the way forward (before releasing the bolt); then press down on the bolt-latch release to let the bolt go forward.

The procedure for fully loading the gun is the same as for half-loading, except that the operation is repeated. Once fully loaded the machine gun is fired by depressing the butterfly trigger.

WARNING

Once fullyloaded, the M2 .50-cal. machine gun maintains a round of ammunition in the chamber at all times. During sustained firing operations, the high temperature of the barrel presents a possible "cook-off" situation. The weapon should always be kept pointed in a safe direction or cleared during breaks in firing. According to Clearing of Live Ammunition from Guns, NAVSEA SW300-BC-SAF-010, the M2 HB reaches cook-off temperatures after a burst of 250 rounds or more.

Figure 3-67.-First round held in feedway by belt-holding pawl.

An emergency situation may occur while firing the M2 or any other belt-fed machine gun: the runaway firing of the gun. That is, firing continues after the trigger has been released. This is remedied by twisting the ammunition belt at the feed slot, thereby causing it to jam and cease firing.

WARNING

In case of a runaway gun, keep the weapon laid on target and DO NOT UNLATCH THE COVER!

The bolt-latch release is unlocked to unload the gun, the cover-latch release is turned, and the cover is raised. The ammunition belt is lifted from the gun. The bolt is pulled to the rear, and the chamber and the T-slot are examined to see that they hold no rounds. After this examination, the bolt is allowed to go forward, the cover is closed and latched, and the trigger is pressed.