The FN M3M (GAU-21) is a .50 caliber (12.7x99mm) single barrel rapid-fire machine gun suitable for rotary-wing aircraft applications providing defensive firepower ranging out to nearly 2000 meters.
The FN M3M was designated by the U.S. Navy as the GAU-21 in 2004 and is currently used by all services within the U.S. Department of Defense (DoD). An evolution of the M3 .50-caliber heavy machine gun, it produces a blistering 1,100 rounds per minute cyclic rate of fire through the use of open-bolt operation and a dual recoil buffer system. Operating independent of either electrical or hydraulic power sources, the FN M3M/GAU-21’s unique soft mount system enhances weapon accuracy and minimizes the firing vibration transmitted to the airframe.