Not sure what you are saying about the 5.56 and the 223 but if you are talking about the bushmaster it will take both. If your gun is designed for the 5.56x45 like bushmasters are then it is designed for the higher pressure 5.56 and has a longer chamber throat that will handle a 223 Remington. In fact a barrel chambered for only the 223 Remington will have 223 REM stamped and not just 223.