The 8x57mm Mauser has been produced in two calibre variants, both called 8×57.

The initial calibre first produces in the late 1800s was a 7.9×57, using a bullet with .318-inches diameter. These were also called 7.9×57, 8x57I or 8x57J. Later the need for a more efficient bullet lead to a change to a .323-inch diameter bullet. These rifles were designated 7.92 or 8x57JS. A spitzer bullet was used, hence the addition of the S in the 8x57JS

If you have an early rifle the larger diameter (modern) ammunition should not be used in the smaller bore barrel. If in doubt to which rilfe you have get the bore slugged by a gunsmith before firing. On some older rifles the barrels were changed to the larger .323 calibre, this was designated with an S stamped on the barrel.