Not a new question, by any means, but a persistent one: when purchasing an AR15, is there a difference between .223 and 5.56? This blog post attempts to provide some insight.
AR-15 .223 vs Mil-Spec 5.56mm Chambers
Get the right chamber for your AR15 to be successfull Even though many manufacturers and specifications say you can use the SAAMI .223 and Mil-Spec 5.56 mm in the same barrel, there are some pros and considers to consider before you plunk down that hard earned cash.