What is a machine gun simulator?

During the 80's, while on maneuvers, the US Army often had available a gas/oxygen type hook-up with tubes and cords feeding into a mock-up .50 machinegun. It sounded loud and real, and never ran out of ammo (until the steel tanks were empty). It was an enjoyable experience; in contrast to real fifties firing blanks WHICH ALWAYS JAMMED. More time was spent "clearing machine guns firing blanks" than playing war (maneuvers).