The FN M3M, designated as the GAU-21 by the US Navy, is chambered for .50 BMG and fires at a rate of 1100 rpm using an open-bolt operation and a dual recoil buffer system. The effective range of the M3M is 2500 meters (approx. 1.5 miles) with a maximum range of 6500 meters (approx. 4 miles). It operates independent of either electrical or hydraulic power sources and has a unique soft mount system that enhances accuracy and minimizes recoil forces transmitted to the airframe. The M3M been approved by the DoD for use with all branches of the military as the Common Defense Weapon System for all current and future rotary-wing aircraft.