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You are absolutely correct Nathan. 5.56 = Both and .223 = .223 only.

yes you can use .223 ammo in a 5.56 but not the other way

Correct … the reason is that ammo stamped 5.56 is loaded to NATO standard and is a much higher pressure round. While virtually the same dimensions they are not interchangeable. It's best just to stick with ammo head stamped the same as what's stamped on your gun.