My very similar looking Drilling is also marked 8,7mm (clearly it’s bore diameter, not it’s groove one), but what exactly did that mean? I fitted a recently made 9,3x72R cartridge down the tube and it almost fit. I asked my gunsmith to do a chamber casting with Cerosafe. Measured close to the land impressions, it was .364”, corresponding to the typical 9,3x72R bore. So although it's marked 8,7mm but it's certainly not a 9.0mm.
I later obtained a copy of the Gun Digest 1955 article “Proof in Modern Germany”, in which Baron von Engelhart stated: “The very commonly used 9,3x72 (now called 9,3x72R), frequently was marked as 8,7 and similar”. Evidently 8,7; 8,75 or 8,8mm descriptions were also used for the same cartridge.
There are also 3 variations on the 9,3x72R chamber, (called E, D and Normal), and, with the correct FL sizer die set from CH4D, it will now readily chamber modern S&B 9,3x72R cases after they have been full length sized and also after their rims have been thinned. I have here cartridge drawings showing the difference between the D and E cases.
Perhaps this will be of help. If you do prove to have a 9,3x72R, the bullets are smaller than 9,3x62 and 9,3x74R, so be caseful about that also. S&B and RWS both make them.
