i agree with what KEH said
5.56 is around 62,000 units of pressure
.223 is a max pressure of around 55,000 units of pressure
(measuring pressures is highly technical and there are several different methods and many variables i give the numbers only for comparison purposes)
additionally the 5.56 chamber allows for greater variance cartridge sizing to allow for better inter-operability with in NATO countries.
the bottom line is they are not the same round they are governed by different standards organizations (.223 by SAAMI and 5.56 by NATO)
US Army Ordnance Officer