8mm=.315, which is neither "Actual" bore or groove diameter of either the 8mmJ or JS, but simply a "Nominal" number. I seem to recall seeing the German S bore was originally called the 7.92x57JS & not 8mm. The marked dia by the proof house is the Bore dia & as Geno so rightly pointed out 7.8 almost certainly designates a J bore while 7.9 would be an S bore. I have an 8x58 Sauer which was marked prior to 1912 under the gauge system & is marked 172.28 which translates to .300". One realizes at this point in time bores were simply marked by the largest plug dia they would accept & the next one up was 156.14 or .310". This therefore does not mean the gun had a .30 cal bbl, but is indeed a .318" 8mm having a proper bore size of around .307/8.
