See response 45. I recently had precisely this discussion with an Old Salt at one of the shops I frequent; he gave me the same answer. Generally, rifles marked specifically 5.56 NATO will take both 5.56 NATO and .223. Rifles marked .223, on the other hand, may have trouble digesting the NATO rounds due to the pressure differential. However, I got the distinct impression that you should be okay - more or less - going either way.
If you're reloading, you can get into trouble with the brass lengthening and causing a bottleneck in a .223 chamber, resulting in very high pressures. Trim your brass and you should be OK. That said, I'd stick with 5.56 chambers...and shoot either ammo.
.223 through a 5.56, not a problem if it’s quality ammo.
5.56 through .223, no way, no how. It’s asking for trouble.
(IMHO)