The United States Army is reportedly developing a new machine gun capable of withstanding higher chamber pressures and allowing for rifle-like bullet velocities out of firearms both lighter and shorter than the M4.
The firearm will use a new breech and bolt design that allows the action to cycle even under extremely high chamber pressures. This added pressure will, in turn, allow projectile velocities to increase, improving terminal ballistics even in compact, short-barreled weapons. In one test, Army researchers achieved a muzzle velocity of 2,900 feet-per-second from a 10-inch barrel using a cartridge holding only 15 grains of powder. For context, most rifles with 10-inch barrels can only push a 5.56 NATO round 2,500 fps, which limits the round’s effectiveness.
“The goal is to get rifle-like velocities out of a very small weapon that is high capacity, that’s either adaptable for room-clearing or confined spaces,” Zac Wingard, a mechanical engineer at the Combat Capabilities Development Command’s Army Research Laboratory, told TechLink. “Like you’re getting in and out of vehicles or a subterranean environment, but also applicable for remotely operated systems, so think like perimeter security or ground robot or even a drone.”
The Army is hoping the technologies can be developed for use both in small, lightweight firearms like the FN P90 and in full-size sniper rifles and belt-fed machineguns. The breech and bolt design would make smaller firearms more effective and would substantially increase the muzzle velocities of larger firearms.
According to TechLink, the U.S. Army’s new 24-inch prototype barrel produced muzzle velocities of 4,600 to 5,750 feet per second. The combustion chamber pressure was increased from 65 ksi to 100 ksi (100,000 pounds per square inch), almost double the pressure seen in the M4 carbine.
“The powder used now in most ammunitions can be tweaked, so it runs at a higher pressure, but the guns can’t handle it,” Alex Michlin, the Army research engineer who invented the new design, told TechLink. “That’s why we designed the new breech, so we can take existing propellant and turn the knob all the way up to 11.”
To accommodate these higher pressures, Michlin developed a bolt that screws into the barrel breech. He also developed a collet that surrounds the cartridge while it’s seated in the chamber, which allows the spent casing to be extracted after firing. The tapered wedges, according to TechLink, reduce extraction forces by 50%, leaving more energy for cycling the bolt.
Velocity matters because, as Clay Martin outlined in a 2016 article, short-barreled rifles chambered in 5.56 struggle to push rounds fast enough (around 2,700 fps) to turn the .22-caliber projectile into a “tissue blender.” If researchers can increase velocities, infantrymen can enjoy the benefits of small calibers like the 5.56 without having to carry rifles with 16+-inch barrels.