Teague

I understand this is an old thread, but your logic is flawed. One could come to this conclusion simply by looking at load data. If pressures between 223 Remington and 556 NATO ammunition were in fact the same, there'd be no issue firing NATO spec ammo in a 223 Remington chamber. But that's simply not the case. NATO Chambers have longer throats to accommodate higher pressure ammunition. Just look at load data for each using same powder and bullet weight. The NATO load data starts off somewhere near max for 223 Remington.