[Nyles]

Actually, the difference is in the leade (the space between the end of the cartridge and where the rifling starts. It's somewhat longer in the 5.56mm NATO, which can lead to reduced accuracy when firing some .223 cartridges out of a NATO chamber, or reliability when firing NATO ammo out of a match-grade .223 chamber.

NATO allows a different max pressure than SAAMI, but that's got more to do with different methods of measuring than anything - the European proof houses give the same max pressure for both. Military cartridges have thicker brass since they're more concerned with reliability than reloadability. SS109 or Mk.262 could potentially lead to higher pressure in a .223-spec chamber, but not so much that it's gonna blow up in your face.