Ordnance Notes --
by Bob Stoner GMCM(SW) Ret.
Mk 2 Mod 0 And Mod 1 .50 BMG/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
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
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.