glassman, I'm not positive, but I believe the 8 x 57 JR is an old round with a .318 diameter bullet, and the available ammo that I have seen is the much more common 8 x 57 JS, which has a .323 diameter bullet. RWS may be the only source left for the JR.
Before you shoot this rifle, mike the bullets and be sure they are not the .323 bullet, and/or cast the rifle bore with cerrosafe and see if it might be the .323 diameter, or is in fact a .318 bore. I wouldn't think that the JS ammo would chamber in a JR, but can't say for sure. I do know that I haven't seen a box of JR ammo in 30-40 yrs. Be careful with what I'm guessing is a very fine old drilling.
I just checked the 8mm Mauser in the Nosler reloading manual. They call the .318 round a (J) not a JR, so I may be completely out in left field, but it's sure worth checking out.
Now I'm wondering if the R stands for rimmed? A lot of the old drillings and double rifles were chambered for rimmed cartridges. A quick look in the chamber will tell you if it is chambered for a rimmed cartridge.
Good luck, Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time