When I studied this topic I concluded that much of the confusion lies in the subject, which oscillates between being the cartridge and the chamber.
I also contacted two manufacturers of .223 REM bolt action rifles, Browning and CZ. I asked if you could shoot 5.56 NATO in their rifle. Browning said NO. CZ said YES.
If you find your back is up against a wall, maybe you have been backing up for too long.