I also heard that .556 has a thicker case don't know if its true or not so if reloaded for a 223 it will create higher pressure because the case capacity is reduced because the brass is thicker?
There isn't any difference beyond normal variation between manufacturers.
http://www.m4carbine.net/showthread.php?t=55149
Other cases, like .308 and 7.62x51, do have a significant difference in thickness between military and commercial brass. A good rule of thumb is to reduce powder charges by 1-2 grains when reloading .308 with military brass.


















































