I'm reliably informed that there is a difference between 5.56 NATO and .223 Remington loading procedures...and that you shouldn't "mix and match" the two. Apparently has to do with different "leade" in the chamber throat, which necessitates a slight (but significant?) difference in bullet seating depth? I'm no expert, obviously, but have been told that you can fire either in a rifle chambered for 5.56 NATO, but that you should only fire ammo specifically loaded for .223 in a rifle marked for that caliber. Y'all talk amongst yourselves...
I remember hearing that...although I am quite certain that 5.56 cases are thicker, allowing for less powder to be placed in the case. IOW, the powder charge in a 5.56 case should be adjusted downward a bit in comparison to .223.