8X57IR and 8X57JR are the same and both use the .318" bullet. The S (.323") bullets are loaded in cases with black primers in Europe to distinguish them from the "J" and "JR" ammunition. All the American factory loads, as far as I know, are loaded with the .323" bullets.

You're fortunate to have such a drilling at your disposal. I have a Sauer Model 26, which is a side lock drilling with separate rifle barrel cocking by means of a lever on the left side of the action. Cocking the rifle lock shifts the front trigger to fire the rifle barrel, and after firing the rifle barrel, or uncocking the rifle lock, the trigger must be returned to shot by means of a tiny button located adjacent to it.