ATK, the parent company of ammunition manufacturers Federal Cartridge Company and Speer, published a bulletin entitled “The Difference Between 223 Rem and 5.56 Military Cartridges.” In this bulletin, ATK stated using 5.56 ammo in a .223 rifle could result in “…primer pocket gas leaks, blown cartridge case heads, and gun functioning issues.”
.223 Remington vs. 5.56 NATO: What You Don’t Know Could Hurt YouHowever, ATK dropped this bomb in the bulletin on the .223/5.56:
“It is our understanding that commercially available AR15’s and M16’s – although some are stamped 5.56 Rem on the receiver – are manufactured with .223 chambers.”
https://bearingarms.com/bob-o/2011/02/1 ... -hurt-you/