To my mind one only has to ask why would a barrel maker in Birmingham and London spend time enlarging a barrel's bore diameter greater than the standard to which all the gunmaker's worked to; or for that case bore and finish the barrel's bore diameter less than the standard? Two reasons come to mind, and those are the barrel maker is getting paid to do it because a gunmaker's customer desires it; and secondly to correct a flaw in the barrel discovered during boring and finishing the barrel to standard (in this discussion we are using .729")
Removing more metal from the bore of the barrel costs the barrel maker more and he is getting paid for making the barrel to standard, going larger results in less profit for him. It therefore seems within reason to assume that as soon as the barrel maker measured that he was where he should be in the bore diameter lapping to "standard" he stopped and polished the bores and send the barrel set on.
Just because the proof house may have had primitive barrel bore measuring devices that they were required to have under the rules of proof for the period of time does not mean that the barrels makers did not have accurate incremental measuring devices. Accordingly, we cannot assume that the proof houses did not have bore measuring devices which could measure the bore in .001" increments, since micrometers were invented in the 1800's. However they were required to use the mandrels to measure the barrel's bore to meet the proof laws at the time, as the following quotes from the 1954 UK Rules of Proof demonstrates.
The following is directly from "Rule 18" of the 1954 UK Rules of Proof, page 8: .....(a) As to an Arm of the First Class---- (according to p.3 SCHEDULE B arms of the "first class" are "smooth bore breech loading arms of 4 bore or smaller discharging shot or bullet"....) "Each barrel shall be gauged at a distance of 9 inches from its breech face for its bore diameter which shall be within the limits set out in Table 1 of Appendix III for the diameter of such barrel (this Table shows that 12 bore guns can have bore diameters of a min of .710" to max of .751" diameters). The bore gauges to be used shall be of the diameters set out in Table 2 of Appendix III(This table delineates for the 12 bore .710", .719", .729", and .740" diameters of bore). Such bore diameter shall be deemed to be that of the largest bore gauge which will enter the barrel to a depth of 9 inches from the barrel breech".
It is evident from reading the above that a shotgun barrel could have measured a max of .729" at 9 inches from the breech and the bore could have measured much larger at 9 1/2" (or wherever along the barrel length) from the breech and meet the 1954 Rules of Proof. I suspect that from time to time this was encountered and the two proof houses had the incremental measuring devices to measure to measure such differences for the full length of the barrels bore.
In today's world of gun manufacturing where measuring a gun bore diameter easily and throughout the barrel, these 1954 rules may be seen as crude, but they worked. The Proof Master and his men had keen eyes and they denied proof many times on the basis of their visual examination.
