They are not the same!!
Please go to:
http://www.thefiringline.com/forums/showthread.php?s=&threadid=15031&highlight=223+vs+5.56
and:
http://forums.ar15.com/Forum14/HTML/001085.html
"ACCORDING TO SAAMI (THE PEOPLE WHO SHOULD KNOW) THERE IS NO DIMENSIONAL DIFFERENCE IN .223 AND 5.56mm CARTRIDGES. THE DIFFERENCE IS IN PRESSURE LEVELS OF THE LOADED CARTRIDGES.
.223 is lower pressure (about 52,000 CUP) and 5.56mm is designated as higher pressure. SAAMI has long stood by their recommendation that 5.56 not be used in rifles labeled as .223 because of the risk of overpressure. It may never happen in your gun, but it's like using 38+P in a revolver designated as 38 only.
There are also differences in chambers. THIS IS ALSO NOT A SAAMI REGULATED DIMENSION. Military rifles and some others use a SLIGHTLY larger chamber to facilitate feeding/extraction, especially under sustained fire and in adverse conditions. Remember, the first and most important thing about a combat arm is RELIABILITY, thus, the larger chamber.
It's really usually only a matter of running a finishing reamer through the chamber after the rough reamer a few more times than usual to get a 5.56 chamber instead of a .223 chamber. THEY ARE THE SAME TOOLS. HOWEVER, some manufacturers use finishing reamers that are slightly larger, thus facilitating faster prodiction and lower tool wear."
Some manufacturers (like Ruger) used to chamber their guns for 5.56 to avoid problems, I don't know now.