M60 machine gun
The M60 was the standard US machine gun during the Vietnam War.
It shoots a 7.62 bullet, an ammo than can reach very long ranges and creates much more damages with regard to the ball used by the M16.
It was designed to be used laying on the ground and operated by two men: one for shooting and the second holding the ammo-belt and reloading it. To solve the problem of holding the belt, soldiers used to tape a can on the gun, but the weapon was lately updated fixing this and other problems.
Anyway, even with those modifications, shooting standing or from the hip was not a task for everyone.