My opinion and take it for what you will. There ain't enough difference to matter. Don't do it now because I shoot my on reloads, but I have shot 5.56 in a .223 marked rifle and no issues. Taken to an extreme with high pressure 5.56 shot in a weak chambered .223 there might could be an issue. I have bought .223 that measured exactly the same as some once fired .mil brass. Held exactly the same amount of water too. Maybe years ago there was a bigger difference I don't know.
Rifles don't blow up with one hot round. Repeated firing of high pressure rounds will stress the steel until it eventually fails.
