You guys are gonna kill me for asking - but why do they write it like that - .223 is not the same as 5.56, and are not really interchangeable (although I think that .223 can, in a pinch, be used in 5.56 chamberings).
Or should I not have asked this question?
.223 inches diameter, 5.56 millimeter.
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.