?????? the 1896 Mauser was chambered in 6.5x55.


Almost every "J" (.318) bore military rifle in German service was rebarrled, or had the throat opened to accept the "S" (.323) bullet adopted in 1905. This includes most G88s as well. Seems there was a war on, and they were not about to let perfectly servicable rifles sit it out simply because of an ammunition change.
Most existing .318 bore rifles were hunting rifles, and .318 bore rifles were made up until at least the late 1930s. You do run into the occasional miltary G88 that wasn't converted, but they are relatively few.