15.8mm or 15.9mm would convert to a bore dia of .622"-.626" which seem mighty small for a 16ga. A nominal 16ga bore of .662" converts to 16.8mm. Might this number be the muzzle diameter of a tightly choked 16ga? 15.9mm is actually about dead on for 19ga.
2-piper may just have a point. I've trudged back thru the Ferlach proof rules and it appears they closely mimic the Belgian rules. The 1st steps of the Ferlach rules were to visually inspect the tube, verify calibre, stamp calibre as well as the proofhouse control number(datastring) and last the act of proof itself. I can't find anywhere where the bore diameter measurement was taken. It wasn't until July 11th, 1893 that the Belgian rules specified that the bore measurement was to be taken at 8.6(6) inches. Hopefully Peter will have some insight here. It would seem that many of the 16 bores are stamped in the 15.9 to 16.2 range with the muzzle diameters closely following the last pass diameter stamp. The few 12 bores I've seen were in the range of 17.2 to 17.8mm. So if any have an Austro-Hungarian sporting arm, I'd be interested in your muzzle diameters and stamps just forward of the flats.
Kind Regards,
Raimey
rse
