RE: Is there a diff. between .223 and 5.56mm?
SAAMI is like the FDA and are easily bought off like many of you have already pointed out. When I first learned there was a difference I called Bushmaster, Olympic Arms and my local gun smith at Gander Mountain. They all said that you could fire .223 and 5.56 from either chamber and that there was no significant difference, danger or risk of damage by doing so in an AR platform. The the Gun smith and the Bushmaster rep did warn against using a 5.56 in a .223 Chamber for Bolt Actions though.... not too sure why extraction and the emergency pressure release might be some things you might encounter in a bolt action.
while oly arms doesnt have the greatest reputation in the AR worldthey do have a lifetime warranty and I dont think that they would have anything to gain by telling customers to use improper ammo...