Smith and Wesson M&P 15 is nice rifle, or if your looking towards more of a deer hunting caliber bump up to a AR-10 (.308 or 7.62 NATO).
Or if you don't like AR look but want the reliablity of combat rifle. Springfield M-1A (M-14) is nothing but bad @zz.
There shouldn't be a issue firing 5.56 or .223 stamped ammo. There is no difference between the two. The only thing you might have a issue with firing stamped 5.56 if it was belted or de-linked machine gun ammo.