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.
Something tells me your barrels weren't nitrided.If the nitrined barrel would last for ever, how come the sig barrels are very often replaced after 10000 rounds?, mine 551 did and so did my shooting buddys 550, both barrels was keyholing after that magig 10000 line.
And both was 1/7 twist.