When the US military adopts a cartridge, it does so under varying and sometimes conflicting requirements. The 5.56 x 45 was developed from the commercial .222 Remington. Another cartridge under consideration was .222 Remington Magnum. The .222 Remington Magnum was actually intended to be the cartridge of choice for the military. The 223 Remington in civilian disguise has the same profile as the 5.56 NATO but is loaded to different pressures (NATO round is higher pressure) and the camber profile in the military cartridge has a longer throat. You can shoot .223 in the military weapons but keep the 5.56 NATO out of civilian chambers, too much pressure. Anyway, this all occurred in 1959.
The .222 Remington was designed to handle game of up to 40 pounds out to 200-250 yards. The current 5.56 NATO is the M855 (US designation) for the SSS109 specification. It fired a 62-grain steel-cored bullet to pass the NATO required to penetrate helmets at long range. This from a machine gun. Lethality in soft tissue was never addressed. Terminal effectiveness (lethality) depends on striking velocity, yaw or fragmentation to cause soft tissue damage. The M855 round effectiveness is severely degraded beyond 150 meters when fired from the M4’s 14.5-inch barrel. Malnourished narrow torsos of the middle east belligerents experience little or insignificant wounding from the M4. Unless the M855 yaws in the target, through and through shoots is the norm. The small thin torsos do not give the bullet sufficient target depth for yaw and tumbling in the tissue medium.
NATO Round & Short Carbines
The NATO round and short carbines like the M4 and MK18 CQBR are not an effective combination. Enemy soldiers therefore continue to pose a threat even with multiple hits. The Taliban, it has been said, “Ignore 5.56mm, respect 7.62mm and fear .50 BMG.” (Small Arms Defense Journal 2012-01-06). A typical mid-eastern tactic is to stay just outside of the effective range of the rifle. They use older Soviet weapons like the SVD and PKM chambered in the 7.62 x 54mm Russian offering a longer standoff range. Hence, the popularity of M14 with frontline troops.
Under pressure from congress to develop a ‘leadless’ the M855A1 with a solid copper base and a steel penetrator in the nose. It is a 62-grain round and leadless. But it has its own problems. It works beyond the pressures of the standard 5.56 NATO round, about 5000 psi, pushing the chamber pressures from 52-55,000 psi up to about 61-63,000 psi with the pressure test round at 70,000 psi. Firearm failure; mostly the bolt is common at about 3,000 rounds with a 50% reduction in barrel life. The round has not been adopted by the Marine Corp or other services.
6.8 x 43mm SPC
Enter the 6.8 x 43mm SPC, a round developed in the Special Operations community. The bore is the same as the .270 caliber and will work within the limitations of current magazines and operate in current rifles with an upper receiver replacement with the new barrel and bolt. The Army is going for a new rifle, carbine and squad automatic in the new round. The new round and weapons are due to start as early as 2021. The Marines are sticking with 5.56 NATO as well as the US European and Asian allies.
Nothing is ever simple when comes to weapons and ammo. Many believe that the 6.8mm SPC offers only minimal performance improvement over the current 5.56 NATO. Congress isn’t happy with two bullets in 5.56 with the Army and the Marine using two different rounds, let alone introducing weapons and new cartridges that are not compatible with anything else used by our forces and our allies. Time will tell.