Frontlander, I load the 8X60 RS for a J.P. Sauer double rifle. This gun won't take the pressures that yours will, but I have a load that uses the 200 grain Nosler 8mm Partition bullet and 59 grains of RE22 for 2450 FPS MV.
The current load shoots like this, R & L barrels @ 100 yards.
When I first got this rifle, I had a couple of pounds of H205 powder, 55.7 grains of which gave 2620 FPS MV with the Nosler Partition 200 grain bullet at pressures mild enough for the double. This powder, (NATURALLY!!) is no longer available, and no other powder will produce as high velocities as did H205 at useable pressures in the double.
BUT, in a Mauser, you are not as limited. I discovered, back when I had some H205, that I could use the SAME CHARGE of H205 and IMR 4350 with 200-grain bullets in the 8X57mm in a Mauser action and get the same velocities exactly. Therefor, I believe you could do the same in an 8X60S in a Mauser, using IMR 4350. I would recommend a starting load of 51 grains of IMR 4350 with a Nosler 200-grain Partition or Speer spitzer bullet and Federal 210 primers. This load should give you +-2400 FPS. Then work up in half-grain increments to a max of 56 or 57 grains of the same powder, which should give you around 2650 FPS with that bullet. You MIGHT even be able to reach 2700 with the 200-grain bullets and IMR 4350! This performance level is close to the old German "Magnum Bombe" load for this round, which was considered equal to the .300 H&H in killing power.
If I am not mistaken, as a young man Finn Aagard once killed a Cape Buffalo with an 8X60S Mauser using factory ammo.
I used 200-grain bullets in my 8X60RS double because the gun was regulated for 198-grain bullets. However, I also tried 150 grain Hornadys, 175 grain Sierras, 185-grian and 220-grain Remington Corelokts in it as well. If you are interested, I can go dig out the data for these which includes chronographed velocities for the various loads. (BTW, the damn double shot 150 and 175-grain bullets as well as the 200-grainers, as far as regulation is concerned. However, my loads were kept conservative, and could be bettered in a bolt action. If I ever come across a bolt job in 8X60S, I may just buy it! Despite the fact that this cartridge originated as kind of an "accident de guerre", it is a good one!
KURT!! That BRNO is fabulous!!
NOTE: IF IT IS MARKED 8x60 AND NOT 8x60 S, MAKE SURE IT WILL SHOOT .323" BULLETS. THE WAY TO DETERMINE THIS IS TO TAKE A FIRED CASE, AND SEE IF A .323" BULLET WILL SLIP THROUGH THE NECK INTO THE CASE WITHOUT HAVING TO BE FORCED. IF IT WILL, THEN .323" BULLETS CAN BE USED. IIF YOU DON'T HAVE AN UNRESIZED FIRED CASE, THEN THE BORE SHOULD BE SLUGGED TO DETERMINE GROOVE DIAMETER. < !--color-->
