The XM312 was a developmental lightweight heavy machine gun developed by General Dynamics in response to the U.S. military's request for a replacement of the M2 Browning, and as a complement and fallback to the XM307 ACSW grenade launcher.
The XM312 is a belt-fed weapon that is fed from either side. It features the same "differential firing" recoil mitigation system featured in the XM307. At the time of cancellation, it was using a rather strange hybrid operating system with recoil-operated feeding but a gas-operated bolt.
With unit-level work and a few parts, it can be converted into an XM307 in under a few minutes. This works the other way around.
In September 2005, the U.S. military began test firing the XM312.
In 2008, both of the ACSW weapons were cancelled, due to the low rate of fire and failure to meet weight goals. Much of the technology from the XM312 was incorporated into the XM806, another attempt to design a lightweight .50 caliber machine gun, but this too was plagued by rate-of-fire problems and ultimately cancelled.
The .338 Norma Magnum LWMMG currently under development by General Dynamics uses a derivative of differential firing called "short recoil impulse averaging," and appears to have finally defeated the rate-of-fire problems with a cyclic rate of 500 rounds per minute.