Minor points:
Most military brass is thicker than commercial brass, giving less case volume and higher pressures. That is true of all the 308 military brass that I've ever checked.
The notable exception is LC 5.56 brass. It has more case capacity than commercial brass that I've compared with, except Federal.
Because of the longer leade of 5.56, you can theoretically make cartridges that will jam into the lands of a 223 chamber. And that would cause high pressure. But those rounds would not usually fit in an AR15 magazine.
When fired in some machine guns. the bolt is so forceful and tight that it shortens the brass by an average of .014". So if you get once fired brass from a machine gun, the shoulder may be a lot farther back that you would like, raising the possibility of case head separation.