I could be wrong too. But with the 1st 1875-1887 London marks on the tubes as a 20B, it would have noted that the 20 bore plug gauge passed the full length and I would say a pretty broad girth regarding the actual diameter. From the 1868 rules for rifled small arms, a 20 bore would have covered the 0.61" to 0.620" range(0.610"<20 bore<0.620") while as a reference the 12 bore is centered at 0.729" between 0.720" and 0.730"(0.720"<12 bore<0.730"). At the moment I can't find any inch equalivent bore diameters(non-rifled) for the 1875-1887 period. So by the 1955 rules the actual diameter of a 20 bore would have been more precisely defined and this fact along with wear would have put the diameter of at least one tube outside of 0.2mm. Also, the point for measurement was changed from full length to 9" from the standing breech. But I think previous marks were still honoured/valid unless there was some alteration and the longarm was in a diffent state than when originally proofed/proved. And it was probably time for a baseline or a benchmark to begin to compare the diameter for the 0.2mm tolerance.
Kind Regards,
Raimey
rse
Last edited by ellenbr; 02/01/09 05:58 PM.
