The M60E3 is a 7.62x51mm NATO medium machine gun that is a scaled-down version of the M60 General-Purpose Machine Gun. It is seen in Rambo: First Blood Part II and is featured so predominantly in the film that it often comes to mind when you think of Rambo's character, much like the compound bow, exploding arrows, and his survival knife.
Rambo pulls the machine gun off of a mount on a helicopter and uses it to shoot up a Viet Cong prison camp, killing about ten people with it and using it to shoot the lock off of the cage holding the POWs. He then rescues the POWs and places the gun back on its mount, and it is used once again during an intense air battle with Colonel Podovsky's helicopter. When Rambo makes his emergency landing at the hangar, he takes the gun off its mount once again and uses it to beat up Ericson. Rambo goes into the command center and begins destroying the surveillance equipment with bullets. Rambo fires the gun empty and discards it like a used tissue. Although it is in the film for a relatively short amount of time, it has become so well known because of how Rambo uses it (one handed with an ammunition belt wrapped around his hand) that it is always thought of with the character and has been used in parodies of the Rambo films, like Hot Shots! Part Deux and UHF.
The gun is also used on many promotional photos and the box art for the film. Although the gun was shown mounted on a helicopter, this probably would not have been the case. Although the M60 and the aircraft M60D version are famous for being mounted on the doors of helicopters, the M60E3 was specifically designed to be handheld, with a foregrip for easier firing. It was also made smaller specifically for the purpose of being used by paratroopers and soldiers during combat as a light machine gun.