Never ever buy an AR-15 in .223. It will jamb with 5.56 ammo. Buy a 5.56, it will handle .223 without a hitch. The guns/ammo are not fully interchangeable like the .308/7.62x51 are.
***Never ever buy an AR-15 in .223. It will jamb with 5.56 ammo.***
Never had a problem with the two different designations for a .223. Never a jam.
My Colt rifle has 5.56MM stamped on the rifle, the box it came in has .223 on it. Same serial number on both the rifle and box.
And I have only been shooting M-16s since 1966, AR-15 since 1970.