[GingisK]

Belay that mass statement! Mr. Ohaus indicates that the military stuff is a bit heavier, especially when I clean both samples out well.

The external size of the cartridges are the same, however. The following is a good reference: http://www.smellysmleshooters.net/ammopressure.htm

[Hazcat]

Here's a link to one disussion on it.

http://www.fulton-armory.com/308.htm

[going hot]

the brass varies by lot number for both military and civilian cartridges.
I weigh the brass, and group them by weight. If one case weighs more than the other, and the outside is the same, then the difference is on the inside, The heavier one actually shoots higher (I find), most likely due to the higher pressure when touched off, due to same charge in a smaller space. The difference can be more than 20 fps out the barral.