@ fromtheplaines
I'm not positive about this, but my understanding is that all of the currently manufactured Boxer primed 8x56R brass is produced by Prvi Partizan. This includes Hornady, Graf's, Buffalo Arms and of course Prvi Partizan.
I doubt that you'll find much difference in case capacity, but its simple enough to check.
Remove the decapping pin from your sizing die, then size and trim your once fired cases.
Now fill the cases with an appropriate substance (some folks use water, I prefer Alliant Bullseye powder), then dump and weigh the amounts from each case. Obviously, the more of each headstamp case you weigh, the better the resulting data will be.
If the weights are pretty much the same, capacity of your cases is the same as well.
If the weights vary from brand to brand, you'll get an idea of how each type should be loaded.
Either way you'll know more than you did before you checked.