The Mk.20 mod.0 automatic grenade launcher was developed upon request from US Navy by its own Naval Ordnance Station Louisville, as a short-range area denial armament for small riverine crafts, which were widely used by the US Navy during the Vietnam war.
Development of a new 40mm automatic grenade launcher started in mid-1966, and it was ready for trials a year later. The weapon was relatively light and soft-recoiling, as it fired a 40x46mm LV (low velocity) grenade rounds, originally developed for the M79 shingle shot hand held grenade launcher. As such, the Mk.20 mod.0 automatic grenade launcher was well suited for installation on the small and light boats, machine guns’ tripods, and other light mounts. In fact, it was possible to fire it off-hand, although the accuracy was dismal and the effective range extremely short. The production of Mk.20 grenade launcher started in 1970 and lasted about a year, with a tad over 1,000 guns produced for the US Navy. The Mk.20 mod.0 grenade launcher replaced in service the manually operated Honeywell Mk.18 grenade launcher. In turn, the Mk.20 was soon replaced in military service by the noticeably more powerful and effective Mk.19 automatic grenade launcher, which fired a more potent 40x53mm HV ammunition to a much longer range.
The Mk.20 mod.0 automatic grenade launcher is an interesting weapon which combines a blow-forward barrel and a simple blowback bolt. Before firing, the barrel is held cocked in its forwardmost position, with its spring compressed. The bolt is in its forward position too, and the fresh round is held by the feed pawls against the bolt face. When trigger is depressed, barrel is released and its spring forces the barrel to slide back, over the ready cartridge, to chamber it fully. Once the barrel is in battery, the firing pin is released to ignite the propelling charge. The subsequent discharge forces the bolt back along with the fired case, and the projectile forces the barrel forward, compressing two sets of spring and sort of counterbalancing the recoil forces.
At the end of the recoil stroke the empty case is ejected, and during the forward movement of the bolt the fresh cartridge is fed into the ready to fire position. After that, the firing cycle can be repeated, providing that the trigger is still depressed by the operator.
Ammunition was fed from metallic disintegrating belts, stored in cartridge boxes. Typical mounts for the Mk.20 mod.0 automatic grenade launcher were pintle or pedestal mounts on boats; some guns were piggybacked above the M2HB machine guns on pedestal mounts to provide more flexible fire options.
The Mk.20 mod.0 automatic grenade launcher basic specifications