I'm looking at purchasing a Colt AR-15 Sporter Model in 223 caliber. The receive is stamped 223 caliber. Can I shoot the NATO version of 223, the 5.56x45 out of this rifle? I'm under the impression I can, but I heard from some that it is unsafe to shoot 5.56x45 rounds out of a 223 unless it is stamped for both. Anyone have experience with this type of question?