It has been my experience that British proof houses have done a poor job of examining chamber dimensions for compliance for the last 80 yrs or so.I have imported many used Brit guns and have found the majority to be undersized. There are a number of reasons for this but most must have been overlooked at initial proof and any subsequent re-proof.The issue is as follows:-
1954 Rules of Proof
Table 1 p 26 clearly states 12 bore forward chamber dia to be Min.0.800"Max 0.810", should be clear but......
Note at bottom of table states" To allow for polishing of chamber after proof dia shall be gauged as if such minimum dimension were 0.003" less". I speculate both proof houses only used a .797" go gauge, I also believe they didnt always check even with that
Thus we can see at least on new production guns, barrels and sleevers a forward dia of 0.797" was permitted (with the presumption that it would be polished up to 0.800")CIP no longer accept this and require chambers to be to finished dimension i.e. .800"-20,3mm
SAAMI picked up on this and have set forward dia min as .797".in their standards
Any Brit gunsmith getting reamers from Brownells or other US sources will end up below the now enforced std Brit CIP dia.
I had Clymer custom make me a 3" 12chamber reamer with .802" forward dia, .005" per inch chamber taper and a pilot after an @ 1/2" forcing cone. It cleans up chambers beautifully.
I must sympathize with Digs second point about lack of a heads up to the trade,effectively that last weeks accepted practice will be prohibited on Monday.
To do that the Proof House would have to admit to not doing their job , at least since 1954, which as we see from gunmans rules isn't going to happen.

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745