Rifles marked 7,8mm are, as I understand it, all .318" bore because the lands are 7,80mm, the grooves 8,07mm and the bullet 8,09mm (0.318"). I had a Louis Knappworst Drilling made in 1931 that was marked in this way and it shot the .318" bullet accurately and to expected ballistics. Apart from the 7,8mm marked 8x57 I (Infanterie) with .318" bore, and the 7,9mm marked 8x57 S, there's also a 8x57R 360 based on an English black powder cartridge. If you search for Tom's Wiederlade-Ecke online, (www.geocities.com) you should find a reprint of the 1983 RWS Wiederlade reloading manual that shows all 3. A Cerrosafe cast will tell you which you have.