Hi, Cousin,
No 8x57 Mauser has a .318" bore size; all 8x57 rifles have a .311" bore size.
As to the old .318" groove diameter (8x57J), no European military rifles made after c. 1905 have the old size, they are all .323", same as the new bullet diameter (8x57JS).
Why do you think your 24/48 (??) has the old groove diameter?
Jim
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