She is correct, the wall thickness of the two cartridges are different.
The 5.56 case has thicker walls to handle higher pressures, meaning the interior volume of the case is smaller than that of a .223. This will alter the loading data used when reloading 5.56 brass to .223 specs.
Wrong again chippy....
http://www.humanevents.com/2011/02/15/223-remington-vs-556-nato-what-you-dont-know-could-hurt-you/
Oh then I guess you missed her link at #112 that reads accordingly:
"All Bushmaster barrels are chambered to accept 5.56mm ammunition and always have been.... so that the 5.56 ammunition can safely be used."