I have weighed some of my own brass and perhaps I just happened to get civilian brass that weighs less but my military surplus always weighs more. I used to have a couple cross section cases that displayed this too. I might make some more after I get a chance to get down to the range to fire off some new loads. Anyway, the rounds differ by pressure, no matter what you or I consider "significant". SAAMI sets the cap at 55k and NATO sets it at 62k. There are significant studies on the cartridge that show my point on this. And I agree that almost every manufacturer uses one of variant 5.56/.223 chambers allowing you to safely shoot both, the one exception I know if is the mini14 target models but you also covered that in your reply. So I guess we are saying the same thing but from different perspectives. The older mini14s used to not have the 5.56/.223 chamber which is why I even bothered to reply to this thread.