W.T..... AR's vs. Bolt guns has nothing to do with it.

The 5.56 NATO chamber and the 223 Rem chamber are different. In some instances that may matter, in some it may not.
Personally I've never had any 5.56 ammo, not chamber in a 223 rifle. At this point, an AR-15 with a 223 chamber would be much less common than a NATO chamber.

http://www.luckygunner.com/labs/5-56-vs-223/