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

Mk 18 Mod 0 Manual 40mm Grenade Launcher

Mk 18 Mod 0 40mm grenade launcher ready for action aboard a PBR during the Vietnam War. Note the large ammunition box on the left, that gun feeds from left to right, and the expended cases remain in the belt after firing. The kneeling sailor has an M79 single-shot 40mm grenade launcher. (Photo: US Navy)

The first attempt to build a fast-firing alternative to the single-shot M79 was the Honeywell Mk 18 Mod 0 multiple grenade launcher. The Mk 18 fired the same round as the M79 launcher, the 40x46mmSR, and not the more powerful 40x53mmSR round used by the later Mk 19 Mod 0. The Mk 18 used a split breech system and was manually operated by a hand-crank. The split breech was actually two pairs of four-lobed star wheels. The tab of its fiberglass-reinforced belt of 24 rounds was introduced into the feedway and pushed through while the hand-crank was turned to index the first round. Once indexed, the gun was set on SAFE until needed.

A good close-up of a Mk 18 mounted on the forward, starboard weapons mount of MST-2 Detachment ALPHA’s Light SEAL Support Craft or LSSC just after its return from an operation. The fabric belt was a fiberglass tape construction that resisted the climatic conditions in Southeast Asia. (Photo: US Navy)

Use was simplicity itself: the safety was set to OFF and the hand-crank was turned. The star wheels drew the rounds into the firing position -- centered between the upper and lower star wheels -- then fired -- and then passed out the ejection port by the rotating star wheels. The 40mm grenades were not withdrawn from the belt like other machine guns; the fired cases remained in the belt.

Rate of fire depended on how fast the gunner could turn the hand-crank. The Mk 18 weighed about 8.6 kg (19 lbs.) and was 56cm (22.5 in) long. Design commenced in 1962 and nearly 1,200 were built between 1965 and 1968. This grenade launcher and its contemporary automatic equivalent, the Mk 20 Mod 0, have both been replaced in service by the more powerful Mk 19 Mod 3 40mm automatic grenade launcher in Navy Special Boat Squadrons.

Mk 18 Mod 0 (Honeywell) 40mm grenade launcher broken down for maintenance by EN2 Ron Simpson aboard his PBR during the Vietnam War. (Photo: Ron Simpson)

2005 Bob Stoner R3