Why a new round? SPC is lauded as the answer to terminal deficiencies found in the 5.56x45mm. This round was initially developed by SOC, or special operations command. The 5.56 NATO has a perceived lack of terminal deficiencies when fired from short barreled weapons like the M4. The 6.8 SPC a.k.a. 6.8 x43mm is a .270 caliber cartridge. Twist rate in 1:12 to as tight as 1:7 inches. Based on the hull of the 30 Remington, a 115-grain bullet can churn up to 2,625 ft/s muzzle velocity. Just for comparison the Soviet Era 7.62×39 pushed M43 was good for 2,421 fps pushing a 123-grain bullet.
M4 Carbine Rifle
Remember it is was the short barreled M4 that the problems where encountered from its 14.5-inch barrel. Terminal performance fell off rapidly. The M4 replaced all submachine guns, most handguns (shades of the M-1 carbine) and the M16A2 as the forwardly issued weapon of the US Army. The Marine Corps issued orders that officers up to the rank of Lt. Colonel and staff non-commissioned officers carry the M4 instead of the M9 sidearm. Though the standard battle rifle is the M16A2 Navy corpsmen assigned to the Marine Corp also carry the M4. The 6.8 sized to fit the standard M16/M3 magazine well. Change the barrel, bolt magazine. The close quarter combat rifle was designed for this cartridge. The 6.8 SPC is being considered for the new squad automatic.
Rifle & Cartridge Specs
According to the Prototype Opportunity Notice posted online, the US Army wanted the new weapons to be able to fire in both semi-automatic and fully automatic modes as well as share the same ammunition magazine. The new rifles had to include a flash hider for minimizing muzzle blast at night time, a removable sound suppressor, and a carry sling. The rifle must be resistant to rust and scratches as well as chemical, biological, nuclear weapons, electromagnetic pulse, and cyber attacks and must be available in some variation of Coyote color, a shade of brown popular with the US military. Finally the new rifles must function in all environments and weather conditions and come equipped with a Picatinny rail for mounting of optics, aiming lasers, and other weapon enhancements.
The current military specification is for the 6.8 SPCII. It has 2.5mm freebore to allow for the higher pressures of military ammo.