Have a look inside the book "Mauser - Original Oberndorf Sporting Rifles" by Jon Speed. On page 275 he shows a hand-written table on barrel dimensions from the Mauser factory files, dated December 9, 1926. It shows the 8mm cartridges with barrel dimension suited for the 8mmI = .318" bullets, but there is a footnote in German below this table. Translated by me it reads in modernized parlance: "The Hunting rifles for the cartridges 8x57I (.318) and 8x60I (.318") get the chamber and throat for the S-type .323" bullets, so that the S cartidges may be used in these rifles." I would not hesitate to use European CIP standard factory loads or comparable handloads in your just marginally undersized, .3215", barrel.