Keep in mind chamber size and cartrige size when talking Rem .223 or 5.56 NATO:
While the cartridges are basically the same except the thickness of the brass, the chambers are slightly different. First off, the 5.56 NATO has a longer leade (distance bullet must travel before engaging the rifling). The extended leade distance is about .025″ – which is a meaningful distance for accuracy based on bullet seating depth. Second, some of the other chamber dimensions are “enlarged” very slightly in order to allow for more reliable feeding/chambering characteristics.
Here is a summary of the differences (positive number means greater in size for 5.56 NATO):
Base diameter.002″Shoulder diameter.0007″Neck.001″Freebore diameter.0025″Base to case mouth.003″Base to shoulder.004″Neck length-0.002Freebore length.025″Throat angle-0.6 degrees