IF I remember right the 5.56 nato is about 5k hotter and the chamber is slightly bigger but thats what I was told.
I think the brass is the same dimensions except the 5.56 is thicker at the base.

Somebody told me a while back that the 5.56 no go gage was the same as the 5.56 nato go gage etc like it was a
couple of thousands longer.