I believe (if not correct me) the 5.56 brass has thicker case walls then .223 Remington same goes for .308 rem. brass and 7.62 NATO brass. .223 rem is not as hot as NATO spec ammo, .223 rem. runs at about 3000 fps where 5.56 NATO runs at about 3250 fps in my AR 15 20" barrel.
You can tell by the head stamp on the case, NATO will have the circle with a cross in it. In a combat situation all NATO country's that run across another country's ammo that find the circle with a cross in it know it's good ammo. Also SAMMI states that firing 5.56mm in a .223 rem. chamber is an unsafe ammunition chamber combination.
My AR 15's love XM193 from Lake City that's box and sold by Federal, good stuff but expensive. I also love the 5.56 brass for reloading for my AR 15's it was plentiful but harder to find now.
Just my .02
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