I knew that there was a difference between military and commercial brass, but didn't realize how much.

I just finished cleaning some .303 British brass, mixed HXP surplus and Remington commercial.

The weight between the two was very apparent, just from sorting.  So I pulled out the scale and took a weight sample of the two.

The HXP weighed out, on average out of 20 cases, at 179.6 gr.

The Remington weighed out, on average out of 20 cases, at 159.1 gr.

I was shocked that there was an average of 20 grains of difference between the two.

And here I thought the difference was that commercial brass was carefully picked up and put into range bag for reloading later while the military surplus brass was hastefully grabbed off of the ground and tossed into a box or ammo can.  Extra weight might be extra sand/dirt.  

mil has a crimp on the primer is what ive read.

That is another difference, but because crimps are easily removed, I don't consider them a major difference in the construction of the brass.