They both have the same demensions , but the 223 Remington is loaded at a lower pressure than the 5.56 military round . It is not reccomended to shoot military 5.56 rounds in arifle chambered for 223 Rem. At least that is what I have read . They just ran a article on this in shotgun news . I found this out the hard way myself , didn't hurt the rifle but blew some cases . Duh