Interchangeable Calibers?

[Da Beak]

Are the 5.56 NATO and .223 Rem interchangeable?

[Whoaru99]

Yes and no....I think.

A .223 cartridge should be safe in a rifle designed for 5.56, but 5.56 ammo may not be safe in all .223 rifles.

I believe it has to do with chamber pressure. True 5.56 cartridges are military and usually are loaded to higher pressure. Also, a .223 rifle may tend to have a tighter chamber/throat for better accuracy than a 5.56 military chamber/throat which I think is just a bit looser to keep cycling reliably under rapid fire/high heat conditions.