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

NOS Louisville Mk 20 Mod 0 Automatic 40mm Grenade Launcher

The Mk 20 Mod 0 was an attempt to improve on the hand-cranked Mk 18 Mod 0 40mm belt-fed grenade launcher. The Mk 20 design was designed by Naval Ordnance Station, Louisville, KY. The Mk 20 used the same ammunition as the Mk 18; that is, the 40x46mmSR. The Mk 20 was a blow-forward operated weapon that fired either single shots or full automatic.

An excellent view of a Mk 20 Mod 0 40mm automatic grenade launcher on one of the first Light SEAL Support Craft deployed to Vietnam. Notice the gun is cocked (barrel is forward). The cocking handle does not move as the weapon operates. A box of 24 linked grenades attaches to the gun cradle. The cover above the feedway guides the incoming 40mm rounds. A star wheel in the feedway moves the 40mm rounds through the gun. The button between the spade grips is the trigger. The rear leaf sight is folded down. (Photo: Fred Schuler)

The belt was fed into the gun from the left as in the Mk 18. The barrel was cocked (moved forward) and the feed pawls indexed the round into firing position. When the gun was ready to fire, the barrel was cocked (and extended). When it was clear and unloaded, the barrel retracted inside the barrel support.

The Mk 20 had paired spade grips similar to the .50 Browning machine gun. When the firing button between the grips was pushed, the barrel moved rearward, chambering the round, and firing it. The barrel was blown forward, which re-cocked the gun and indexed the in-coming round.

The Mk 20 used the steel links of the Mk 19 automatic grenade launcher. Cyclic rate was about 250 to 275 rounds/minute. The gun was roughly the same size and weight as the Mk 18, except that it was not as bulky. The receiver of the Mk 20 was a sheet metal cover that attached to the backplate and barrel support by Allen-head screws. The Mk 20 has been phased-out of the Special Boat Units in favor of the more powerful Mk 19 Mod 3 automatic grenade launcher.

Specifications, Machine Gun 40mm Mk 20 Mod 0:

Caliber ………………………………………………… 40x46mm SR
Weight ………………………………………………… 26 lbs.
Length ………………………………………………… 27 in.
Width ………………………………………………….. 9 in.
Height …………………………………………………. 7 in.
Mode of Operation ……………………………….…. Semi- or Full Automatic
Feed ………………………………………………….. Left Hand
Maximum Range ……………………………………. 400 meters
Maximum Effective Range …………………………. 350 meters
Muzzle Velocity ……………………………………… 230 feet per second
Barrel Length ………………………………………… 12 in.
Cooling ……………………………………………….. Air





© 2005 Bob Stoner R4