The M79 and XM148 grenade launchers were single-shot, and the repeating T148E1 grenade launcher was unreliable, so a request was made to China Lake engineers. SEAL Teams were pleased with the resultant pump-action grenade launcher, since the tubular magazine held three 40x46mm grenades, and so with one grenade in the chamber, four grenades could be fired rapidly before reloading. In fact, a skilled operator could fire four aimed shots before the first one landed.
"American captain Dennis Muchlstedt working two phones while attempting to contact units of 9th Infantry who are on patrol in the South Vietnamese-Cambodian border area", by Larry Burrows, Vietnam, 1970 - Vietnam War
A Navy SEAL, carrying a Mk23 5.56mm Machine Gun (Stoner 63), moves through deep mud as he makes his way ashore from a boat, during a combat operation in South Vietnam in 1970. (U.S. Navy photograph by PHC A. Hill)