I was in my local gun shop this morning and looking at the Mossberg MMR Hunter (AR-15). The clerk said that it was chambered for 5.56 X 45 (NATO) only and .223 Remington could not be used. I have never heard of this before. I know that you should not use 5.56 NATO in a gun chambered for .223 but have always thought you could use .223 in any gun chambered for 5.56 NATO. What is going on here? I have checked Mossberg site as well as a couple of reviews on this rifle and none mention that it cannot be used with .223 rounds. All say 5.56 NATO/.223.