The gauge system was a rather crude method of marking rifle bores in that from .450" (51.05 ga) down they only seem to have marked in increments of .010".
118.35 gauge converts to .340" (8.64mm) bore size. Next size up the ladder is 51.05 @ .350" 8.89mm). Your actual bore size could be anywhere between these two dimensions. A 9.3x72R barrel I have was nitro proofed in 1913 & is marked as 8.8mm.
I have a 1907 8x58 Sauer barrel which is marked 172.28 (.300"/7.62mm) It has a groove dia of about .321". I forget the actual bore size but is larger than .300" yet smaller than .310" (156.14 ga). These gauges are calculated on the exact same basis as are the regular shotgun gauges. All gauges from 50 (.453") & larger ( lower numbers) are listed as whole number gauges with diameters carried to three decimal places.
Beginning with .450" down through .300" the diameters are in .010" increments with the gauge number being listed to two decimal places.
A barrel proofed for smokeless powder should have a crown over N stamped on it.
