The "standard" 8mm Mauser is 7.92x57, however, there is an earlier 8x60, and an 8x60 S. What he HEARD as 8x60 Magnum MAY have been 8x60 Mauser. However, unless there are caliber markings ON the gun (now required by importers, but was not always so) uncle REALLY needs a smith to check it for the right caliber.
8x60 Mauser is scarce, but CAN be found- Prvi Partisan still loads it. But not an 8x60 Magnum.
What we have here is... failure- to communicate.