I am a late reader of all these posts about 2 inch chamber 12 bore guns and it may be that my input here is irrelevant or of no interest. But it seemed to me as I have just briefly read all 13 pages of this post that there were questions in some readers/posters mind about the proof pressure differences (if any) between the proof of 2 1/2 inch chambered 12 bore guns and 2 inch chambered 12 bore guns via the two proof houses in the UK. I just pulled out my copy of the "RULES OF PROOF 1954" ---"Rules Regulations and Scales made in the month of October 1954" and turned to page 20 (of 45 pages) titled APPENDIX II---"for the proof of barrels and the arms of the first class". This appendix chart shows that for Provisional Proof the loading for both 2- 1/2 inch and 2 inch chambered 12 bore guns use the identical proof loads: 266 grains of Tower Proof (T.P.) powder and 1- 1/4 ounces of no. 6 shot. However the Definitive Proof loads for the 2- 1/2 chambered 12 bore guns and the 2 inch chambered guns are different; the Definitive Proof load for the 2-1/2 inch chambered 12 bore gun is 178 grains of Tower Special Proof (T.S.P.) powder and 1- 11/16 grains of no. 6 shot, while the Definitive Proof load for 2 inch chambered 12 bore gun is 170 grains of T.S.P. and 1-5/8 ounce of no. 6 shot.

This data above does not answer the question if both 2-1/2 inch chambered and 2 inch chambered 12 bore guns are proofed to the same proof pressure but it give reasonable expectations that the UK provisional proof pressures could the same or even more for the 2 inch chambered gun; and in the case of definitive proof the pressures are very similar.

I also noted from this appendix (as referenced above) that 2-3/4 inch, 2-1/2 inch and 2 inch chambered 12 bore guns all were proofed with the same Provisional Proof load as listed above; and I was more than a bit surprised at this.

Kindest Regard;
Stephen Howell