.223 and 5.56 are basically the same ammo. The only real worry would be what barrel your gun is chambered for... A 5.56 barrel can shoot .223 ammo just fine, but there is potential for issues if you're trying to fire 5.56 ammo from a .223 chambered barrel. I can't remember the exact reason, but I think it has something to do with possible differences in how far the bullet is seated in the different cartridges which can lead to over-pressurization in the chamber.