The U.S. Navy and Marines have decided to standardize on the GAU-21 machine-gun system for all their heavy helicopters. The GAU-21 uses a Belgian M2M .50 caliber machinegun, instead of the older U.S. made M2 (Ma Deuce). The M2M has less recoil and fewer malfunctions (cook offs). The M2M also has longer barrel life and a higher rate of fire (1,000 rounds per minute, compared to 700 with the older M2 weapon). The GAU-21 is able to accurately hit targets 6,700 meters (21,000 feet) away, partly because the weapon generates less vibration than the M2. The GAU-21 machine-gun costs $60,000 each.
U.S. helicopters also use the M2M as door guns, but in the navy and marines, these will largely be replaced with the GAU-21, even in the marine UH-1N (the refurbished twin engine UH-1 they have been using for decades).
Three years ago, German CH-53G transport helicopters in Afghanistan had their armament (two 7.62mm machine guns, mounted on the side doors), increased with a .50 caliber (12.7mm) machine gun on the rear cargo ramp. The U.S. Navy and Marines first developed this system four years ago, calling it the GAU-21. The larger caliber machine-gun provides more effective firepower when dealing with ambush situations, or anyone on the ground firing at you. The Germans also plan to replace the door guns with 12.7mm machine-guns.