Quote Originally Posted by snowgetter1
I have fired thousands of mixed .223 and 5.56 Nato through Mini-14s, AR-15, M-4s, M-16s, and a Stevens 223 Bolt without a problem. I know there is a slight differemce but have never understood why people get so wrapped up in it. Not trying to be offensive just stating my thoughs and experience. Never has been a concern.
Fired in a SAAMI spec .223 pressure barrel, 5.56 NATO loads have shown pressures in the 85K PSI range. That's bordering on a proof load. The good news is that most .223's made these days are not cut to SAAMI spec.
The only difference I have seen is the thicker Military brass leads to higher pressures quicker when reloading max. loads.
You can't make the assertion that mil brass is thicker than commercial. It varies by manufacturer completely unrelated to mil or commercial. That is why you sort by headstamp and work up loads separately for each different one.

http://www.accurateshooter.com/cartridge-guides/223rem/ Check out the case data about halfway down the page.