As I see it then the change from simply marking the bore as 12 to .729 actually accomplished Nothing. My understanding was that a bore marked 12 was in proof until it would accept the 12/1 gage (.740"). Under the .010" rule the .729 mar4k3ed bor would be in proof until it accepted a .739" gage while under the .008" rule it would go out of proof when it accepted a .737" gage.
In any case the gun could have been proofed with a bore diameter of .736" & would go out of proof (Depending upon the rule it was proofed under) with .001", .003" or .006" enlargement.
I guess I had been mis-led down through the years but I had thought the reason for the change to the decimal marking was to allow a given amount of enlargement from the "Actual" proof diameter rather than from a "Nominal" diameter.
I have seen "Continental" guns marked in metrics which for a nominal 12 gauge carried a marking other than 18.5mm or 16.8mm for a 16 gauge. Apparently They got it right & marked the actual measured diameter & then give you that constant 0.2mm leeway for honing.
