RDHD45,
Since it's a "Schoenauer", it's likely 6.5x54 MS. The same cartridge was sometimes called 6.5x53,depending on the maker, and was called 256 MS by the Brits.The rifle should be marked 1903. If it is 1908,10,or something else, then it shouldn't be 6.5. If it has a scopemount dovetailed into the reciever, the markings may be gone.
Mike