Britain has joined the U.S. Navy and Marines and installed the GAU-21 machine-gun system for their heavy helicopters. Thus Britain is equipping its Sea King commando helicopters with the M3M (called the GAU-21 in American service). The GAU-21 uses a Belgian M3M .50 caliber machinegun, instead of the older U.S. made M2 (Ma Deuce). The M3M has less recoil and fewer malfunctions (cook offs). The M3M 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.
British and U.S. helicopters also use the M2 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 GAU-21 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 are replacing the door guns with 12.7mm machine-guns.