My 9.3 x 72R drilling barrel is stamped 8.8mm with a line beneath & 72 below the line. beneath that is 6 13 over 240. My understanding of this is it would have been the 240th arm proofed in June of 1913. It is nitro proofed for a 12.7gr KMG (196 Grain Copper Jacketed Bullet). Shot barrels are 16ga & simply carry a Crown/N & the word Nitro, no load data.
Remember on these caliber markings the 8.8mm is bore diameter, not the groove or bullet diameter so they will always be smaller than the nominal caliber designation.
I have a 8x58R Sauer proofed in 1908 & it's bore is marked by the old gauge designation of 172.28. This denotes .300". In all likelihood it was just shy of accepting the .310" gage ( 156.14 gauge). It was Nitro Proofed for a Steel Jacketed Bullet which would have been of the .318" diameter.
As stated previously a chamber cast is the way to go. On the 9.3x72R it is my understanding that loads were increased significantly & carried a higher pressure for those carrying Nitro Proof. It has also been my understanding that with these newer loads rims were thickened to prevent them chambering in the older black powder proofed guns. I believe that smokeless proof became mandatory in 1912, prior to that date it was optional.
