My 16/16/9.3x72R hammer drilling was the 240th gun proofed June 1913. It is marked 8.8MM/72. Also have a 16/16/8x58R drilling which has the bore size marked with the old gauge size, 172.28/59. This one was proofed in Feb 1907. Both have Nitro Proofs.
The 172.28 gauge works out to a .300 cal, but my understanding is that at this point they were marked with the largest size plug which would enter the bore & not necessarily exact bore size. The next plug up would have been the 156.14 gauge (.310") & the bore now measures just below that size. It is likely real close to what it was at proof. Note that both have the chamber length marked. The 9.3mm was marked for copper jacketed bullet & the 8mm for a steel jacketed one. The 9.3 was marked for a 12.7 gram bullet (about 196 grains) with powder charge not stated & the 8mm was marked for a 2 gram charge (about 31 grains) of flake rifle powder with the bullet weight not given, presumely the standard bullet, which was about 200 grains.
The definition for "Caliber" was established in muzzle loader days & "True Caliber" is defined as Bore size. Thus we refer to a .308 Winchester as a "30 Caliber" rifle, the .308W desingnating Which 30 caliber rifle it is. The original naming of what we today normally call the 8MM Mauser was 7.92x57 , the 7.92 being .3118" or the bore size. The M14 which sired the .308W is known as the 7.62 Nato, 7.62mm = .300" & not the .308 which Winchester put on it, probably in order to avoid confusion with so many 30 calibers already existing.
