Ordnance Notes -- by Bob Stoner

Mk 2 Mod 0 and Mod 1 .50 Caliber MG/81mm Mortar

A Mk 2 Mod 1 mortar/machinegun mount (aka Mount 52) aboard a PCF (about 1967-1968). This gun has the 100-round box tray and is set-up for right hand feed. The recoil cylinder for the mortar is above the barrel. Note the prominent ventilated basket.  This is a guard to prevent injury to the gunner when the  mortar  recoils when fired.   The mushroom-shaped knob in the background is the mode of fire selector for the mortar: DROP or TRIGGER fire. (Photo: Dave Pendergrass RD2

These 81mm mortars were unique to the U.S. Navy and U.S. Coast Guard small craft of the Viet Nam and post war-era. The mortar itself is entirely different in design from all other mortars in U.S. service. This mortar is designed for direct and indirect fire. The difference between the Mk 2 Mod 0 and Mk 2 Mod 1 are mostly cosmetic and that the Mod 1 mounts an AN/M-2HB .50 Browning machinegun above its recoil cylinder and a 100 or 400 round ammunition box on its right side.

The mortar itself is mounted on a very robust tripod and uses clamps to control traverse and elevation angles. Unless fitted with NO FIRE zone mechanical stops, the mortar has 360 degrees of traverse and -30 degrees of depression and +71.5 degrees of elevation. Its rate of fire is 18 rounds/minute at 45 degrees elevation in DROP FIRE mode and 10 rounds/minute in TRIGGER FIRE mode. Sights for the mortar are attached to the left side of the elevation arc. Weight of the Mk 2 Mod 0 was 593 pounds; the weight increased to 677 pounds in the Mk 2 Mod 1 (with machinegun). Range of the 81mm (direct) was 1,000+ yards; (high angle, indirect) was 3,940 yards. Maximum effective range of the .50 Browning machine gun was 2,000 yards; maximum range was 7,440 yards.

The Mk 2 was developed by the NavWepSta Crane, Indiana, in the early 1960s to provide offshore patrol boats with a light weight direct and high-angle fire weapon that could engage both surface and shore targets. It was adopted by the USCG in 1962 where it was first mounted on their large WHEC cutters in the Atlantic and Pacific. These mortars were used to fire illumination flares to aid commercial and military aircraft forced to ditch at sea. Tests in the Caribbean showed the 81mm mortar illumination round was more effective than the 3"/50 gun's star shell. Not only was the illumination of the 81mm round better, but it fired at a higher rate and had less fouling problems than the 3"/50.

In mid-1964 the USCG recommended the fitting of a .50 Browning machinegun in "piggyback" fashion above the mortar's recoil cylinder. The prototype was built by the USCG at its Curtis Bay, MD, yard and it worked very well. The mortar's tripod mount was more than adequate for taming the .50 Browning's recoil. In late 1964, the Navy fired the Mk 2 Mod 1 at its Dahlgren, VA range. The Mk 2 Mod 1 was successful in its tests. Two Mk 2 Mod 1 units were then taken to sea aboard USCG 95-foot cutters for demonstration and operational evaluation. Both units passed with flying colors.

The Mk 2 Mod 0/1 was deployed by the hundreds aboard many kinds of craft in Viet Nam. A short, but by no means exhaustive list would be: APLs and YRBMs (non-self propelled barracks barges); USCG 82-foot cutters; USCG 95-foot cutters; Navy 50-foot coastal patrol craft (PCF); Navy 75-foot fast patrol boats (PTF, "Nasty"-class); Navy 95-foot fast patrol boats (PTF, "Osprey"-class); Navy patrol gunboats (PG, "Ashville"-class); miscellaneous riverine craft which were mostly converted LCM-6 landing craft: MON (monitor); CCB (command and control boat); Zippo (flame thrower boat); ASPB (assault support patrol boat); HSSC (heavy SEAL support craft); and advanced tactical support bases such as SEA FLOAT/SOLID ANCHOR (Nam Can) and BREEZY COVE (Song Ong Doc).

Typical ammunition used with the .50 Browning was armor piercing incendiary (API), incendiary (INC), and armor piercing incendiary tracer (API-T). It was linked two API, two INC, and one API-T.

Ammunition for the 81mm mortar was the M-43 series HE; M-362 series HE or TP; M-374 series HE; M-301 series ILLUM; M-57 series FS or WP smoke; M-375 WP Smoke; Mk 112 Leaflet; Mk 115, Mk 133 to Mk 135 Chaff; and Mk 120 APERS. Fuzes were either point detonating, proximity, or mechanical time -- except for the Mk 120 which had none. Ranges of the mortar projectiles varied by the addition or removal of propellant bags (8 maximum). All illumination (ILLUM) and Chaff rounds used mechanical time fuzes. Proximity or VT (variable time) fuzes were used with the M-375 WP Smoke and M-362/M-374 HE rounds. All other rounds except Mk 120) used point detonating fuzes.

The Mk 120 Mod 0 APERS (anti-personnel) round was specifically-designed for Navy and USCG use. The round made the mortar into an 81mm shotgun which fired 1,200 13-grain steel flechettes (resemble nails with fins) that were effective to 600 feet. The APERS round had no bursting charge or fuze. Instead, the flechettes were launched by the gas produced by the burning propellant bags clustered about its tail fins.

The Mk 120 round resembled an old-fashioned potato masher. It was approximately 13 inches long, a warhead with 1,200 flechettes that was 81mm in diameter by 5 inches long, with a 30mm diameter by 7-inch long tail boom. The tail boom contained the M-34 primer and M-6 ignition cartridge along with the fin assembly and eight M-2A1 propellant increment charges clipped between the fins. There was a 6-petal shaped drag ring 7-1/4 inches from the nose of the projectile on the tail boom.

When the APERS round fired, the burning propellant gases forced the round out of the mortar tube. During the time it was confined by the 81mm warhead, part of the gas was directed against a lead plug in the bottom of the warhead. The plug would melt and allow gas pressure to build-up in an internal chamber in the base of the warhead. The gas pressure would push against the bottom of a piston that had 1,200 flechettes stacked in two packs of 600 above it. The expanding gas against the piston forced the flechettes through the plastic nose cap where they were free to spread out. The drag ring ahead of the fin assembly slowed the carrier projectile after the flechettes had left.


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