You can use 5.56 in MR223. Chamber in those are ".223" per CIP specification, not per SAAMI*. Difference is that CIP specification is same for .223rem and 5.56 NATO. My old MR223 had over 5000 rounds of 5.56 NATO ammo on it and probably have at least 2000-3000 more since I sold it.Thnx for the informative reply, I guess I might confused with another AR that I have. If I have mr223, by cghanging barrel to 5.56 will give me ability to use 5.56 and .223 or anything else required. I am in Canada and did not see 5.56 model here.