The XM312 is a heavy machine gun

Heavy machine gun

The heavy machine gun or HMG is a larger class of machine gun generally recognized to refer to two separate stages of machine gun development. The term was originally used to refer to the early generation of machine guns which came into widespread use in World War I...

 derived from the XM307

XM307

The XM307 Advanced Crew Served Weapon was a developmental 25 mm belt-fed Grenade Machine Gun with smart shell capability. It is the result of the OCSW or Objective Crew Served Weapon project. It is lightweight and designed to be two-man portable, as well as vehicle mounted...

 25 mm autocannon

Autocannon

An autocannon or automatic cannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannons often have a larger caliber than a machine gun . Usually, autocannons are smaller than a field gun or other artillery, and are mechanically loaded for a...

 and chambered for the .50 BMG

.50 BMG

The .50 Browning Machine Gun or 12.7×99mm NATO is a cartridge developed for the Browning .50 caliber machine gun in the late 1910s. Entering service officially in 1921, the round is based on a greatly scaled-up .30-06 cartridge...

 cartridge. It was designed in response to a request by the U.S. military

Military of the United States

The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.The United States has a strong tradition of civilian control of the military...

 for a replacement for the aging M2HB heavy machine gun, and as a complement to the heavier XM307 Advanced Crew Served Weapon.

It is capable of being converted quickly into an XM307 with a small number of parts and a few minutes of work at the unit level (and vice versa from the XM307).