If the military did consider the 6mm bullet the way to go I suspect the 243 Winchester would likely loose out to another case configuration. Personally I am a little bias towards the 6mm Remington. Suppose the longer 6mm Rem. cartridge may be an issue in the action length required and may create issues with carrying capacity of ammo.

The carrying capacity of ammo seems to be a very good argument for sticking with the 223 Remington. Another valid argument for sticking with the 223 Remington might be that our goal is to incapacitate the enemy not necessarily to kill them. A wounded solder may account for 3 solders effectively incapacitated. As one could make the argument that 2 additional men may be required to care for the wounded.