So a bore between .729" and .739" (I think 18" from the breech?) would be marked '12', and if marked '12' would still be "in proof" if honed up to .739" (I think)
This is correct, except that it is 9" from the breech. It was done with a plug gauge. I quote from the 4th edition of the 'Notes on the proof of shotguns and other small arms' "
At proof if a plug gauge of 0.729" diameter (but not one of 0.740") will enter the bore to a depth of 9", that barrel is at present marked .729" and under the 1925 Rules of Proof would have been marked 12 - and so on for the other bore sizes"
Whether this is 9" into the barrel strict, or barrel plus chamber - I believe is the whole assembly measured from the breech face.
