I have received a slew of questions—many from first time AR-type rifle buyers—about the .223 Rem. and the 5.56x45 mm NATO cartridges. Can I shoot 5.56x45 mm NATO in my .223 and vice-versa? Are these the same cartridge?
Externally, the two cartridge cases are identical. The main differences are that 5.56x45 mm NATO operates at a higher chamber pressure (about 60,000 p.s.i. versus 55,000 p.s.i. on the .223 Rem.) and the 5.56’s chamber is slightly larger than that of the .223 Rem. Also, the throat or leade is longer in the 5.56x45 mm chamber. What does this mean? You should not shoot 5.56x45 mm NATO out of a rifle that is chambered in .223 Rem. And be aware that some .223 Rem. ammunition will not reliably cycle through some AR-style .223 Rem. rifles, but it usually does. As a matter of fact, I have not encountered any difficulty with current .223 Rem. loads cycling through a 5.56 mm AR-style rifle.
Generally, most AR-style rifles are intended to accept 5.56x45 mm NATO and are marked “5.56,” while some are marked “5.56/223.” In the case of an AR-style rifle marked “223”, check with the maker as it may actually have a 5.56 chamber. American Rifleman covered this topic at length with an article by Field Editor Bryce M. Towsley.