B.Dudley,
It will take a little more research to say for certain which ammo to use, but 7.8x57 is the normal mark for the German cartridge 8x57I(Infantry). The rifle might accept 8x57IS(Infantry Spitzer). The I cartridge uses a .318" bullet and the IS uses a .323" bullet.The IS would normally be marked 7.9 or 7.92x57, but there was sometimes enough overlap that IS ammo can sometimes be used in an I rifle. Even though you didn't say which action your rifle is built on, American commercial 8mm Mauser ammo is loaded to be safe in such rifles(although a little anemic). If you fire one of these cartridges in your rifle and a .323" bullet will easily go into the neck of the fired case, IS ammo will likely be useable.I'm sure more information can be given if you provide photos,including clear photos of the Proof marks(likely found on the bottom of the barrel).
Mike
