Good info. Going to inject a little different perspective. Look to the chamber also.
5.56�45 NATO Chamber OAL: 2.550
.223 SAAMI Chamber OAL: 2.410
Wylde Chamber OAL: 2.445
The tighter chamber on the SAAMI spec .223 along with the usually higher pressures 5.56�45 rounds are loaded to is what causes the danger in using 5.56 NATO in a gun chambers for .223 Remington. Pressures can spike up to 15,000psi.
The Wylde chamber almost splits the difference between the SAAMI .223 Remington and the 5.56�45 NATO. This means a lower pressure spike since its not as tight as the SAAMI chamber but a shorter jump to the rifling than the 5.56�45 chamber has. Thats the difference between a 5.56�45 NATO, .223 Remington, and .223 Wylde chambers. Its not overly complicated or this crazy weird stuff, just OAL differences that have an effect on safe pressures for a chamber.
Chamber specs are here also.
http://www.againsttheberm.com/2012/11/223-sami-nato-and-wylde-chambers/
