It has just been leaked that the gun used was the M240, officially the Machine Gun, 7.62 mm. The M240 is the US military designation for the FN MAG (French: Mitrailleuse d’Appui Général, English: general-purpose machine gun), a family of belt-fed, gas-operated medium machine guns that chamber the 7.62×51mm NATO cartridge.
The M240 has been used by the United States Armed Forces since the late 1970s. It is used extensively by infantry, most often in rifle companies, as well as on ground vehicles, watercraft and aircraft. Despite being heavier than some comparable weapons, it is highly regarded for reliability and its standardization among NATO members is a major advantage.