Just picked up a used Colt Match HBAR rifle. The lower says 223. But the barrel markings between the front sight and the brake are: C MP 5.56 NATO 1/7 HBAR. I understand the C is for Colt and MP is Mueller Brass. So my question is: Do I have a mismatched upper? Caliber goes by the barrel markings, right? Any idea what I've got?