5th Batt has a very good point: the 8x56R in this case is the 8x56R KROPATSCHEK. It is emphatiaclly NOT the 8x56R Mannlicher 30M or M.31 (which are the same cartridge but are NOT the KROP round).
And I thought Metric was supposed to make things simpler!
BTW, there is also an 8x56 MS, which is RIMLESS. Again, NOT the right one!
The cartridge you need is difficult to find, brass is enough to scare you out of a year's growth.... but it is an easy one to FAKE.
What you do is get a rifle and get your grubby little mittens on a single ORIGINAL cartridge. You will notice that the BASE of the cartridge has a substantial BEVEL to it. This is not simply cosmetic; the Bolt-face of your Rifle is contoured to REQUIRE this bevel. I have 2 Krops; one is looser than the other, but both NEED this bevel, one rifle more than the other.
NOW you hit the link to the Trade-Ex website, go to "products", go to "Reloading Supplies" and order yourself a bag or 2 of 8x50R LEBEL brass.
When the Lebel brass arrives, you will note that it has a very slight bevel to the rim. That is because it is modern brass; the original stuff had a much greater bevel.
So you make up a .320" spud so the necks don't collapse, chuck the Lebel brass into a lathe and re-cut that Base Bevel so that it matches the one on your original cartridge.
You now can load the Lebel brass into your Krop rifle and it will work, although it WILL be short. This all is possible because the French started devlopment on their Lebel Rifle..... with a Krop Model 1874 chambered for the Gras cartridge which had similar bevel!
You need a .326" bullet of 247 grains and there ain't no setch of a animule..... so you get a mould for the 225-grain Mauser & Commission 8mm slug from Mid-South Shooters' Supply and cast your own.
Original charge was 70 grains of Black, this being the very LAST Black-powder military cartridge to be introduced. BARNES gives 28 grains of 4198 to produce 1670 ft/sec with a 175 jacketed slug. Personally, I would stick with CAST because they WILL fill the bore if you smack them on their base the right way.
I know that our friend Talquin (on this forum) is loading for this rifle; see what he says.
I think I would try to run one with about 12 grains of Red Dot and the cast bullet, see what that does.
If you could get the right bullets, you could duplicate Factory performance with 26.6 grains of SR-4759, duplicate original pressures as well.
Good luck!