M60E4

From the Metal Gear Wiki

A long mainstay of the U.S. military, the M60 machine gun was introduced in 1957 to replace the Browning Automatic Rifle and M1919A6, providing a large amount of suppression fire capability thanks to its belt fed ammunition system. While the original weapon was frequently replaced by M240s due to reliability concerns by many branches of the military, the M60E4 features a number of improvements over the original. In MGS4, the M60E4 can be modified in four ways. It's also one of the few weapons that makes Snake scream like Rambo when he starts firing at enemy soldiers. It is also the main light machine gun the PMC troops use