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Infantry Small Arms / The Warfighter

General Dynamics XM312

Prototype Heavy Machine Gun (HMG) [ 2004 ]

The XM312 made up the infantry heavy machine gun portion of the now-canceled XM307 Automatic Grenade Launcher

Authored By: Staff Writer | Last Edited: 05/12/2021 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

At one point, the General Dynamics XM312 was being developed to replace the venerable and tenured - yet aging - Browning M2HB heavy machine gun in service with the United States military. Design work was headed by General Dynamics as part of the Joint Services Small Arms Program beginning in 2000 which ultimately yielded a working prototype in 2004. The XM312 was to serve alongside the proposed XM307 "Advanced Crew Served Weapon" (ACSW), a 25mm belt-fed grenade launcher. The XM312 was to become the 12.7mm (.50 BMG) heavy machine gun component in the new endeavor as both were completed with commonality of parts allowing each system to be converted to the other with some modification by the crew. In this fashion, the combined weapon system would prove logistically friendly to the US military in the long term. However, both the XM307 and XM312 were cancelled as the US military refocused its efforts on procuring their tried-and-true Browning M2 machine gun and Mk 19 automatic grenade launcher - more cost feasible solutions at the time.

Undoubtedly, the XM312 was a futuristic-looking weapon and the next logical evolution of the heavy machine gun platform. The weapon's physical characteristics included a smooth-contoured receiver sporting a molded shroud. The barrel protruded a distance away from the shroud and noticeably lacked a muzzle brake. Ammunition was fed via a belt which was held in a hard box fitted along the left side of the receiver, ejected through a port at the right side. Sighting was through a flip-up assembly at the rear of the receiver or through (optional) optics allowing for much improved ranged accuracy. The weapon would be typically fitted to a heavy-duty adjustable tripod mounting or a vehicle pintle. The entire weapon system weighed a hefty 52lbs and featured a running length of 61.4 inches with 36 inch barrel in place. A standard operating crew was two personnel - one to manage the firing function and the other manage the ammunition supply, help to clear stoppages and change out an overheated barrel as required.©MilitaryFactory.com
The XM312 initially relied on a gas and recoil system of operation. The gas actuated the bolt as the recoil action managed the feed. When this proved unreliable in testing, it was deemed to make the weapon fully recoil-operated (as in the Browning M2). The action allowed for a cyclic rate-of-fire of 260 rounds per minute to be achieved with 40 rounds through sustained action before requiring a change of barrel. Maximum range during testing reached 2,000 meters.

As in the XM312, the General Dynamics XM806 - another 12.7mm heavy machine gun development appearing in 2009 - failed to unseat the famous Browning M2 series. This development was cancelled in 2012.©MilitaryFactory.com
Note: The above text is EXCLUSIVE to the site www.MilitaryFactory.com. It is the product of many hours of research and work made possible with the help of contributors, veterans, insiders, and topic specialists. If you happen upon this text anywhere else on the internet or in print, please let us know at MilitaryFactory AT gmail DOT com so that we may take appropriate action against the offender / offending site and continue to protect this original work.


Service Year

United States national flag graphic
United States


Prototype Heavy Machine Gun (HMG)

National flag of the United States United States (cancelled)
(OPERATORS list includes past, present, and future operators when applicable)
Fire Support
Capable of suppressing enemy elements at range through direct or in-direct fire.

Overall Length
1,560 mm
61.42 in
Barrel Length
914 mm
35.98 in
Empty Wgt
41.89 lb
19.00 kg

Basic Iron; Optional Optics


Recoil-Operated; Rotating Bolt; Belt-Feed

Rotating Bolt
System utilizes internal mechanism to lock the breech or rear barrel assembly prior to firing.
(Material presented above is for historical and entertainment value and should not be construed as usable for hardware restoration, maintenance, or general operation - always consult official manufacturer sources for such information)


0.50 BMG

Sample Visuals**

Graphical image of a 12.7mm / .50BMG machine gun / rifle round
Rounds / Feed

200-round belt
Cartridge relative size chart
*May not represent an exhuastive list; calibers are model-specific dependent, always consult official manufacturer sources.
**Graphics not to actual size; not all cartridges may be represented visually; graphics intended for general reference only.
Max Eff.Range
6,560 ft
(1,999 m | 2,187 yd)

XM312 - Base Series Designation for advanced prototypes.
XM307 - 25mm Automatic Grenade Launcher Prototype on which the XM312 was based on.

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