Barrel lemgth is really critical in a .223 rifle. The .223 depends on bullet yaw and fragmentation which will not occur if rhe bullet's striking velocity is below 2,700-2,750 feet per second. The army found that the percentage offailores if the M4 carbine were significanly higher with M4 carbines (14.5 inch battrls than with M16A2 rifles (20 inch barrels). THey have seriously considered replacing the 14.5 inch M4 narrels with 16.5 inch barrels and will probably do that if the 6.8X43 (.270 caliber) cartridge cannot be adopted because of funding limitations. Army testing has shown that the 6.8mm is more effective than the .223 at all impact ranges.