i ask yesterday if i could fired 5.56 in my new carbine because the lower is stamped 223,i didnt take time to look at the barrel witch is marked 5.56,now i know,anyway i called colt just to see why they stamped the lower 223 and the guy i talked to told me they stamped that way so you would know that 223 could also be fired in the carbine,so if anyone has one like this now you know.