The basic different is the rifles chamber. The 5.56 has a longer lead giving the bullet more jump to the lands. This longer jump allows for the 5.56 ammo to be loaded to higher pressure.
223 load data is tested at a Max Average Pressure of 55K psi. True 5.56 load data is tested at a Max Average Pressure of 62K psi.
As far as I know there is only one source of True 5.56 pressure tested data, that would be Ramshot/Accurate data found here.
http://www.ramshot.com/wp-content/up...ec_1-23-14.pdf
All other data even "Service Rifle" data is tested to 223 pressures of 55K not 5.56 pressures of 62K.