cyphertext makes a good point about reading the manual, I say also take another step and look at the barrel, it should be marked .223 or 5.56 Nato. The difference is in the throating of the barrel, and sometimes in the degree of looseness in the chamber. The 5.56 nato chambers are a tad looser, and the rifling starts a little further down the bore be able to chamber a variety of nato rounds. The nato rounds are also usually loaded to higher pressures than .223. Most but not all AR-15 are chambered in 5.56 these days, but as I said check the barrel markings to be 100% positive