Originally Posted by
Malamute
I believe some do. The real difference is the throat, not the chamber, as I understand it. Ruger has always said their 223 guns were safe to fire any 5.56 rounds in, both their mini-14s and bolt action 223's. In any event, if a particular gun does have the minimum spec 223 chamber/throat, a throating reamer can be rented and open it up fairly simply.
The 223 designation is probably more oriented towards sport shooters who have little or no knowledge of a 5.56 or what differences there may be.
I shot 5.56 ammo in my CZ 527 without any problem. Its likely throated in such a way that it doesn't matter what loads you shoot. Theres differences in other calibers. Browning 45-70s are known to have short throats compared to some other factory guns. It only matters with some bullet profiles, and is simple to open up the throat if need be.