There is volumes of data about the differences/similarities between the .223 and 5.56.
I hope that the concern voiced (ok, typed) by 'dabbson' have been some what answered.
I an sure that many, many more pages of thoughts will come to light about the same worries.
I have been loading for some 50 years and gone through a period of 'faster and faster' and learned many things not to do (swelling the cylinder of a S&W Model 28 was one). In my old (older) age I have settled down to wanting to hit my target (mostly plastic screw-on pop lids - for you yankees pop is your soda or the British fizzy drink), not blowing up the hill sides. I no longer get to hunt as much as I would like, so the search for that perfect load in each rifle and pistol continues to elude me.
Point is with out entering into design and configuration, the .223 will run and well in an AR that is marked as being 5.56. And if one were to be faced with a serious need, the 5.56 will run in a .223 chambering. BUT I would do it only as an experiment or great need and then very seldom. Physically the two rounds are just about the same. Load wise there is a difference. Chamber wise and throating wise, they are much different.
The original post is by a man someplace around Chicago without a place to test/play with his toys. He was concerned about many things including safety. If he were to ever get to the Amarugias, South of KC, I would be please to shoot with him.
As I stated in my original babel, I do love these types of back and forths.