XM174 grenade launcher

The XM174 is a mounted automatic 40mm grenade launcher heavily based on the M1919A4 machine gun and the M79 grenade launcher but fed from an ammo can. It was used in the Vietnam War as a tripod mounted crew-served weapon and as a weapon mounted on various vehicles and aircraft.
The ammunition was held in a drum canister with a maximum capacity of 12 40mm rounds. The launcher was also capable of semi-automatic fire (one round per pull of the trigger).
U.S. Air Force Security Police personnel in Vietnam were also issued the XM174 automatic grenade launcher which was often used on perimeter defense posts and heavy weapons vehicles.