A stock M60 is used in tandem with another one. Each rifle alternates three second bursts on a target. A belt has 180 rounds. After two belts, barrels are changed, allowing the barrel to cool off.
In combat, the troops usually just fired until the tracers started veering off, then they changed the barrel.
The barrel of a .50 cal aircraft gun (light barrel) will melt and sag after about 10 seconds of cyclic fire. Then the armor piercing rounds will exit through the side of it.