I can't believe that some guys (even handloaders) can't see the problems that a 5.56 military round might create in a sporting rifle .223 Rem. chamber. Notwithstanding the pressure differences, the risk of a bullet being pinched overly tight by a case that is jammed into a chamber that is too short is one that should be appreciated. A person that has the time could pop the bullet out of a 5.56mm military case and trim it to correct length. I personally would remove the primer puncher from my sizing die and re-size the necks too. Why Remington standardized on a minimum (at best) case length who knows.