The XM312 .50-caliber machine gun from General Dynamics is scheduled to begin replacing the M2 machine gun in 2005.
The XM312 .50-caliber machine gun from General Dynamics is scheduled to begin replacing the M2 machine gun in 2005.
 
The XM312 will accept belt-fed ammo from either the left or right.
The XM312 will accept belt-fed ammo from either the left or right.
 
A soldier in an armored vehicle and armed with a .50-caliber machine gun guards the perimeter in Baghdad. The Browning machine gun started out as an aircraft gun in 1921 but was modernized and adapted to ground use in 1932 as the M2.
A soldier in an armored vehicle and armed with a .50-caliber machine gun guards the perimeter in Baghdad. The Browning machine gun started out as an aircraft gun in 1921 but was modernized and adapted to ground use in 1932 as the M2.

The new weapon from General Dynamics, Falls Church, Va., is based on prototypes of the company's XM307, a rapid-fire grenade launcher scheduled to see service in 2010. Changing only the barrel and five other parts converts the grenade launcher into a machine gun. But the Pentagon didn't want soldiers to wait for a more accurate and stable weapon, so the Army is deploying XM312s two years from now.

The XM312 is a lightweight, low-recoil gun that mounts on vehicles and tripods and fires standard .50-caliber cartridges, including tracers, armor-piercing incendiaries, and sabot light-armor penatrators. Rounds feed from the left or right using the same deteriorating metal belt as the M2 uses.

The gun weighs just 43 lb, including tripod, much less than the Browning M2's 128 lb, but still heavy enough to require a two-man team. Its peak firing rate is only 260 rounds/min, much less than the M2's 600 rounds/min. The relatively slow rate of cyclic fire or peak firing rate makes the new weapon ineffective against fast-moving and airborne targets. But the XM312 should have a slightly higher practical rate of fire (the rate of fire a gun can sustain for an indefinite period without damage) than the M2's 40 rounds/min. Range will also improve, going from the M2's 1,500 to 2,000 m. A planned magnification and night-vision scope will let GIs take advantage of this longer range.

The 312 has less recoil thanks to an open-bolt, out-of-battery-action design. That means that when firing, the gun barrel moves, not the bolt. In essence, the gun barrel and several other subsystems are being pushed forward by a powerful spring when the round fires. So recoil must first overcome the inertia of these moving assemblies, then compress the spring before the user feels a kick.

General Dynamics and the Army are tight lipped on some of the new weapon's engineering specifics such as muzzle speed, barrel length, and the materials being used.

Once the XM307 grenade launcher is deployed, the Army will supply kits for converting it in the field to an XM312 machine gun and for turning the 312 into the 307, giving front-line soldiers flexibility in choosing weapons.

The 307 fires 25-mm air-bursting munitions using a computerized laser-based system that sends target data to an IC in each round. Soldiers laze a target, move a computer-generated reticule on the weapon's fire-control system over the target, and fire. The round is programmed to explode at the proper range. So even if a target ducks out of sight, the round will explode overhead.

The 307 should weigh about 50 lb, compared to the 144-lb MK19 40-mm grenade launcher it is replacing. And MK19 rounds are lobbed into high arcs. Army officials say the 307 fires rounds on a flatter trajectory, boosting their velocity and letting a soldier put three rounds on a target at 1,200 m in the same time it takes the MK19 to send just one. But 307 rounds will each cost about $22 versus $16 for a 40-mm MK19 round. A .50-caliber round, on the other hand, costs about $2.50.

-- Stephen J. Mraz