Ciruits is right, the 5.56 chamber is slightly "larger" than the .223 SAAMI chamber. And he is also correct in saying all Bushmasters, except their DCM rifle, is chambered in 5.56. The next question was does it matter. That answer will usually depend on who you ask. From what I have gathered, usually it will not be dangerous, but shooting ammo which is 5.56 in a .223 chamber could potentially be dangerous. The chances of something going wrong may be remote, but there is a chance. Aside from that, the larger 5.56 chamber does help aid in reliable feeding and extraction.