Toby;
Very good discourse on this. I will say that while I have little doubt that the better makers did try to hold their bores close to nominal I also seriously doubt that makers of lower priced guns paid quite as much attention. Thus they probably continued using reamers which through wear & resharpening came in a bit below nominal. This could easily have resulted in a 12 being proofed as a 13/1 & with only a thou or two of honing room before going out of proof.
Rob;
On the 16/1 I get .669" to .677" but either way your .674" would still be in proof.
.677" is nominal 15 gauge size & 16/1 is halfway between 16 & 15 dropped (Not rounded) to three decimal places. Thus (.677" + .662") / 2 = .6695" & with the .0005 dropped becomes .669". A 16/1 proofed bore should thus accept a .669" plug gage to the required depth but not accept a .677" one.
