An M 79 it is. It looked like a short, very fat, single shot break open shotgun, in 40 mm. It fired a wide range of rounds, including HE, smoke, willie pete, CS gas, Frag; and for close in work, either a shot shell round or a flachette round. Very nasty weapon, but usefull to have around. In Vietnam, each rifle squad was assigned one. I believe it has since been replaced by the M203 gernade launcher ( the one that mounts under the M 16's barrel ).
By the way, it was also called a "blooper", in either case, it was named for the sound it made when fired.