GI_Brat, most people shoot mil spec 5.56mm through .223 Rem chambered weapons and never even know the difference. Will you have a problem? Probably not, and if you did it would probably not occur for many thousands of rounds, until perhaps near the end of the barrel life. Remember that the weapon is designed for a certain chamber presure. Exceeding it by 20% for thousands of rounds is not something I will do. All it would take is one barrel with some kind of flaw in it to ruin your day. And since most mass produced barrels cost the manufacturer $20 or so, I doubt their is any real quality check of individual barrel structural integrity. SAAMI classifies 5.56mm military spec ammo through a .223 Rem chambered weapon as unsafe. That's good enough for me.
www.saami.org/Look in the 'Publications' menu, under 'Unsafe Arms and Ammunition Combinations.'
It should be noted that you have to look on the barrel to find the chamber spec, not the lower receiver. For instance, the Bushmaster Varminter lower receiver is marked 5.56mm, even though the upper is chambered SAAMI .223 Rem. The Varminter I saw did not have a barrel engraving stating this, it was only stated in the catalog (or it was not visible under the handguard).
I am not an engineer who designs weapons, and I have no knowledge of the degree to which a .223 Rem chambered AR-15 can take the stress of firing mil-spec 5.56mm ammo over the life of the weapon. I will listen to those in the profession who, conservatively no doubt, say it is unsafe. After all, my kids will probably own my rifles some day