XM307 & XM312

XM307 and XM312 were two weapons developed as part of the US Military's cancelled Objective Crew Served Weapon (OCSW) project, intended to supplement or replace existing stocks of M2 heavy machine guns and Mark 19 grenade launchers. The XM307 was also known as the Advanced Crew Served Weapon (ACSW), and fired a 25x59mm computerised HEAB round; a lower-velocity 25x40mm version of this round is used by the Heckler & Koch XM25 and was used by later versions of the equally ill-fated Heckler & Koch XM29 Objective Individual Combat Weapon. The XM307 was able to be converted to the .50 BMG XM312 configuration or vice versa by swapping a small number of parts, in a operation taking a trained crew less than two minutes.

The system's recoil-offsetting system used a moving barrel which was released from the rearward position to travel forward as the weapon fired, forcing recoil to arrest the forward momentum of the barrel before it could have further effect on the weapon. While this resulted in high accuracy and low felt recoil in a system much lighter than the Browning M2, it also resulted in a low rate of fire (260 rpm); this and issues with meeting the system's 40-pound weight goal led to the project's cancellation. The current US project for a lightweight M2 supplement, the XM806, uses some of the technology seen in the XM312. The only surviving part of the XM307 program is the 25x59mm high-velocity round, which is planned for use in Barrett's XM109 Objective Sniper Weapon or "Payload Rifle."

The XM307 and / or XM312 can be seen in the following: