Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.

Mk 4 Mod 0 60mm Mortar

CWO Shepherd of Detachment ALPHA and the crew of their MSSC at Rach Soi. The Navy Mk 4 Mod 0 60mm mortar is on the starboard weapons mount. Note the barrel cover and barrel within. The right hand controls are the elevation brake handle (on right cradle trunnion) and firing mode selector handle (just to the edge of the photo). (Photo: Gary Stubblefield)

The 60mm Mk 4 Mod 0 naval mortar was the little brother of the 81mm Mk 2 Mod 0 naval mortar. The 60mm round was smaller and produced less recoil than the larger 81mm. It was designed for use on small craft like the PBR, LCPL, and MSSC. Like its larger brother, the Mk 4 was capable of indirect (drop) fire or direct (trigger) fire. The mortar was a self-contained weapon that could be installed in any mounting that could accept the standardized (large pintle) of the .30 and .50 BMG gun cradles.

The mortar was served by two crewmen:

• One was the loader who prepared the round for firing by removing the safety pin and lanyard. He loaded the round into the muzzle on command. He also set the fuze of the illuminating rounds with a fuze wrench.

• The other crewman was the mortar gunner or mortar captain. He set the controls of the mortar, aimed, and fired the mortar. He told the loader what charge to set for the mortar rounds; that is, mortars extend or shorten their range by adding or removing propellant bags called “charges.” He also told the loader what setting to put on the mechanical time fuze of the illuminating rounds with the fuze wrench.

The mortar used four types of ammunition:

• High Explosive (HE) M49A2E2. Length 11.61 inches, weight 3.07 lb., explosive filler Composition B, Range – 2,000 yards (maximum), 1,000 yards (effective). It is used for anti-personnel fires.

• White Phosphorous (WP) M302E1. Length 14.28 inches, weight 4.02 lbs, filler is plasticized WP filler with bursting charge, Range – 1,650 yards (maximum). It is used for screening (dense white smoke) or casualty producing from the burning WP particles.

• Illuminating (ILLUM) M83. Length 14.28 inches, weight 3.7 lbs, uses a magnesium parachute flare to light up battlefield at night. Ejected by the action of the adjustable mechanical time fuze, the flare burns for approximately 25 seconds and descends at about 10 feet per second, while producing 145,000 candlepower. The illuminated area is about 100 yards in diameter. Range – 1,000 yards (maximum) at 800 feet altitude (maximum)

• Target Practice (TP) M69. Length 7.72 inches, weight 3.07 lbs. It uses the M4 or M5A1 ignition cartridge for short range practice. It contains no bursting charge.

The controls of the mortar are relatively simple and straight forward. Beginning on the left side is the barrel. The barrel contains the inner 60mm mortar barrel and the outer barrel cover. Between the two is the counter-recoil spring. The barrel terminates into a box-shaped collar with the trunnions that attach it to the cradle. The cradle pintle is inserted into the socket of the weapons mount.

Attached above the left trunnion is the sight assembly; below the trunnion is the train brake, train pointer, and train bearing scale.

A second box in back of the trunnions contains safety catch, trigger, and elevation/train adjusting handle. Inside the hollow box, when its sheet metal covers are removed, is the beech cap for the mortar barrel, the firing assembly, and coupling. The coupling attaches the forward and rear recoil cylinders that protrude from the rear of the box.

On the right hand side by the trunnion is the elevation brake handle. The second handle in back of the trunnion (on the box) is the firing mode selector handle (trigger or drop fire).

In field use, some enterprising gunners made an unofficial “Mk 4 Mod 1” conversion by fitting the mortar with a 7.62mm M60 or M60D to the mortar like the larger .50/81mm Mk 2 Mod 1 version.

The Mk 4 mortar was often over shadowed by its larger 81mm companion. However, it was in continuous use by Naval Special Boat Squadrons through the mid-1990s.

Mortar, 60mm, Mk 4 Mod 0 Specifications:

Operation: Hand; muzzle loading, drop (indirect) fire or trigger (direct) fire.
Ammunition: M49A2E2 HE, M302E1 WP, M83 ILLUM, M69 TP
Overall length of gun: 54-1/2 inches.
Overall width of gun: 14 inches.
Overall height of gun: 57 inches.
Weight of gun: 135 pounds.
Rate of fire: 10 rounds per minute (trigger), 18 rounds per minute (drop).
Effective range: 1,850 yards (max).  Cooling: Air.

A close-up view of the Mini-gun and gunner on Detachment ECHO’s LCPL. The Mk 4 Mod 0 60mm mortar is in the foreground. Note the controls on the left side: sight assembly, train brake handle on cradle, safety catch, trigger, train/elevation handle, and forward recoil cylinder. (Photo: John Engstrom)

© 2005 Bob Stoner R2