So I bought a CMMG Ar-15 today, and the guy I bought it from says it is chambered in 5.56/.223. I've looked all over this thing for a stamping, all over the barrel, and the upper in general. The lower is stamped 5.56/.223 but that doesn't matter. What's the easiest way to tell short of having to buy something? If that's what has to happen I'll just shoot .223 out of it.