I dont have a copy of the 1954 Rules of Proof in front of me but I can recall procedure and process.
The forward end of a 12 bore chamber should be .800" dia min (max .810) when finished (*see note below)This is constant regardless of length.The chamber should taper at a rate of .005" per inch of length.
A plug of the minimum dia should enter to the nominal length i.e 2.5002 for chambers marked as 2&1/2" or 1&1/8th oz.It should not enter to a depth of more than 2.550"
NOTE: A Gunmaker may submit a gun up to .003" undersize to allow for final polishing. I interpret this as only applying to new guns or rechambering jobs with the onus clearly on the submittor to polish this out after proof.
General observation: Proof inspection of this has been sporadic to say the least.I have had 80 year old Greener guns that would only accept .800 plug to 1&3/4".Chambering reamers were relatively difficult to make and I suspect were resharpened causing a loss of dia over time,also it should be noted that standardization only took place immediately prior to WW1 in UK.
Hope this helps
