Gentalmen,
i'm a combat experinced machine gunner , having fired the M60 , M73 , and M2hb in combat , all three are air-cooled , like all air cooled MGs they have a safe burst limit (20 to 25 rounds for the M60 and M73, 5 to 10 rounds for the M2) as long as the gunner can control his burst within these limits , an air-cooled MG will fire all night without melting the barrel.