Jtplumb,
Maybe. The measurements were made using gauge rods or pins. If the barrel is marked 118.35 (8.64mm) that means the 108.49-gauge(8.89mm) rod would not pass through the barrel, but the 118.35-gauge rod did pass. This can result in a .25mm variance in bore diameter. The groove diameter then is twice the groove depth (which can also vary) plus whatever the actual bore diameter is. The way to find the actual groove diameter is by driving a lead ball or slug into the barrel and measure it across largest diameter (an uneven number of grooves complicates this). Barrels marked 108.49 often have groove diameters of .362-.363". This is under the original 1891 Proof law. In 1911 improvements in the law were made, resulting in bore diameters being expressed in millimeters (in .1mm steps) and adding a number for case length, also in mm. This improvement made matters a little clearer but didn't address case dimensions other than length, nor shape. Accordingly, the same mark can appear on rifles chambered for different nominal calibers. Therefore, these marks do not show the nominal cartridge, only the bore diameter and case length. With these two numbers, we can guess what the cartridge is and sometimes the guess is correct. It was not until the Proof Law of 1939 that it was required to clearly mark the guns with the commonly used name of the cartridge it was chambered for. Are we confused yet?
Mike
