The Fabrique Nationale M3M, also known as the GAU-21, is a belt fed, .50 caliber aircraft gun. It is a modernized version of the Browning M3, a heavily modified version of the M2 Browning intended for aircraft use. It fires significantly faster than the M2 at 1200 rounds per minute via a mechanically boosted feed mechanism. The M3 was heavily used in the 1940s and 1950s on fighter jets until it was replaced with 20mm Cannons, and saw significant action with American pilots during the Korean War. The M3M is currently used by the United States Marine Corps on the UH-1Y Venom under the GAU-21 designation.
— In-game description
The M3M is a purchasable vehicle customization in Battlefield Hardline. It is available as a weapon upgrade for Armored SUVs, Counter Attack Trucks, Transport Helicopters, Attack Helicopters, and Gunboats, and is the second most expensive vehicle upgrade in the game behind the Anti-Air Armory, costing $83,000 to unlock for each vehicle class.
The M3M fires significantly faster than the HMG but not as fast as the M134 Minigun; however, it does more damage to vehicles than both weapons and it is very accurate. The M3M is a superior option to the HMG and Minigun if the player is primarily combating enemy vehicles, but it is less effective against infantry than the Minigun.
However, it's primary downside is that is will overheat much faster than any of the weapons making aircooler a preferred option.