223 and 5.56 (nato) "Vive la différence"

The differences can be boiled down to; Two "Tangible" Differences and One recommendation. 

A) Visual difference: 5.56 Case has a Primer Crimp/Swage.. 223 uses the Standard Crimp Pocket. B) Internal dimensioning of Barrel Cartridge Chamber/Pocket .. The Main One that matters is the: Free-Bore Length.  223 is 0.025" .. 5.56 is 0.050". (25 thou. vs 50 thou.)Personal Recommendation:  Safe to fire both cartridges in a rifle chambered for 5.56 NATO.  Only fire 5.56 in a Rifle chambered for 5.56 Nato. Main reason; the 223 shorter "free-Bore" may cause a Higher Pressure build up, if used to fire a 5.56 Nato cartridge.