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When the Navy ramped-up its participation in Viet Nam in 1966 and 1967, it found there was a huge demand for small arms of all kinds. The 7.62mm NATO cartridge had been adopted as Department of Defense standard in 1957. However, the Navy had large stocks of perfectly good caliber .30 weapons remaining from World War 2 and Korea.
One way to put a lot of 7.62mm NATO machine guns in to action was the conversion of the caliber .30 M1919A4 Browning machine guns to 7.62mm NATO. The converted guns were designated the Mk 21 Mod 0. These conversions were very common aboard the converted landing craft of Task Force 117 (Mobile Riverine Force) and its ships [the self-propelled barracks ships (APB) and non-self propelled barracks ships (APL)].
Ammunition used by the Mk 21 Mod 0 was the same as that used by the M60 general-purpose machine gun (GMPG). This ammunition came in a steel can of 200 rounds. The ammunition was linked four M80 Ball rounds to one M62 Tracer round in M13 links. Getting the linked 7.62mm ammunition to function in the Mk 21 with the M13 link was an interesting exercise because the M60 and Mk 21 guns feed in opposite ways.
The M13 link used by the M60 is a strip-out, disintegrating type of link. That is, the loops that hold the cartridge are open on the bottom. A tab on the second loop snaps into the extracting groove of the cartridge case to retain the case in the link. Cartridges inserted in the M13 links hold the belt together. When a cartridge is stripped from the link, that link falls off and is discarded. When loaded into the M60, the two loops are positioned in the feedway, open side towards the gun's feed tray, and against the cartridge stops. Forward movement of the bolt strips the cartridge forward and out of the link to fall in front of the bolt. The bolt continues forward to chamber, lock, and fire the cartridge in the barrel.
The Browning machine guns all use pull-out type disintegrating links. Browning caliber .30 links have solid loops. Cartridges inserted in the links hold the belt together. Friction holds the cartridges in the links. The Browning has a different feeding cycle than the M60. When the gun is cocked, the Browning's extractor pulls the cartridge from the link and feeds into the T-slot of the bolt as the bolt moves to the rear. The cartridge is chambered on the forward stroke of the bolt and the next round is picked-up for feeding. This is exactly opposite of the M60.
In order to make the M1919A4 in caliber .30 gun into the Mk 21 Mod 0 in 7.62mm NATO gun, Naval Ordnance Station Louisville had to:
Install a 6-inch flash suppressor on the end of the barrel jacket.
Replace the .30 barrel with a 7.62mm barrel.
Modify the bolt and feed cover to feed the 7.62mm ammunition.
Add a second belt holding pawl to position the belted 7.62mm ammunition.
Add a spacer block in the feedway to help guide the shorter 7.62mm round and act as a round stop to prevent the use of caliber .30 ammunition.
Stamp the receiver sideplate with: "Machine Gun, 7.62mm, Mk 21 Mod 0."
When the Mk 21 Mod 0 was loaded with the belted 7.62mm ammunition, the cover was opened, the open side of the links were placed on the feedway face-up (that is, towards the operator) with the single loop leading, the extractor was snapped over the first cartridge in the belt, and the cover closed. This was done with the bolt in the forward position. When the cocking handle was pulled to the rear, the extractor pulled the first round out of the M13 link and positioned it in the T-slot of the bolt. The extractor switch cam, riveted to the left wall of the receiver, pushed the extractor down to move the cartridge into firing position. The bolt then went forward to chamber the round into the barrel. At the same time, the extractor switch cam lifted the extractor off the chambered cartridge to pickup the incoming second round as the bolt went into battery (that is, all the way forward and locked). The gunner then pulled the trigger at the back of the receiver to fire the gun.
The difference in feeding operation between the M60 and Browning guns required that the Mk 21 gunner had to remove the ammunition from the ammunition boxes that were issued for the M60 GPMG. He discarded the two 100-round boxes and bandoliers that were inside. He then linked the two 100-round belts together and repacked his ammunition in the same box with the single loop leading and the open side of the link up.
The Mk 21 Mod 0 machine gun was mounted in a cradle that could either accommodate it or the caliber .50 AN/M2HB machine gun. The cradle mounted an ammunition tray what held either 200 rounds of 7.62mm linked ammunition or 100 rounds of caliber .50 linked ammunition. For Navy use, these cradles were equipped with armored shields to protect the gunner and the ammunition.
Machine Gun, 7.62mm NATO, Mk 21 Mod 0 Specifications:
Operation: Recoil, belt-fed, full automatic only. Ammunition: Ball M-80 and Tracer M-62 in links M-13; linked 4 Ball to 1 Tracer Muzzle Velocity: 2,750 feet per second. Overall length of gun: 41 inches (47 inches with flash suppressor). Weight of gun: 33 pounds. Cyclic rate of fire: 400 to 550 rounds per minute. Effective range: 1,100 yards.