pretty shure the only difference between a 223 and 5.56 is a 5.56 has a longer throat for heavier bullets and case are thicker walled giving it a higher std rateing for pressuse
if you really want to do this
i would say use a throat remer or a std 5.56 remer on a 223 and set your head space again with the barrel nut
if went this much trouble i would just do 223 AI or 22-250
their is not going to be a difference if you hand load a 223 vs 556 might get more out of 223 being that it might have very very small more case capicty
military 5.56 rounds are military ammo not very consistant loads and have a crimped primer which is a pain to reload