A more pressing question—will an AR-15 with a 5.56 marked barrel cycle quality .223 cartridges? Or should I stick with ammo marked ‘5.56’? The headspace isn’t identical.
And, is Eugene Stoner some kind of genius or what?
Yes. It’s actually not the headspace that’s different. Now they say that you shouldn’t shoot 5.56 through a barrel marked .223 but .223 through a 5.56 barrel is fine.
“A more pressing question—will an AR-15 with a 5.56 marked barrel cycle quality .223 cartridges? Or should I stick with ammo marked ‘5.56’? The headspace isn’t identical.”
Firing 5.56×45mm NATO from a chamber with a shorter .223 Remington leade can generate pressures in excess of SAAMI maximums.
It’s the other way around.