Other way around...556 is too hot for 223.
good catch Sir...more coffee needed i reckon...
5.56 NATO mil-spec uses a slightly faster powder. .223 chamber dimensions, where the bullet nose is spaced to the throat of the rifling, is a couple thousandths tighter than the 5.56 chambering.
This means pressures can spike dangerously in a .223 chamber when shooting mil-spec 5.56 ammo.