Dan: at the risk of repetition, and being flamed
1. The published pressure for the B&P 12g 2 3/4” Competition One 7/8 oz. @ 1160 fps is 7252 psi. The average increase in Bell's study was about 700 psi. Under the 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula for 2 3/4 tons is 8,120 psi, 3 tons is 8,960 psi.
Unfortunately, the B&P hulls after firing do measure a full 2 3/4"
2. Buried in Bell's study is a suggestion that lengthening forcing cones can somewhat mitigate the rise in pressure when using long for chamber length shells; ie. slightly more volume in which the gas expands.
3. AN EXPERT can lengthen the cones, and the resultant wall thickness, because the angle of the cone is more acute than the external taper of the barrel, will be no less than the pre-lengthening end-of-chamber wall thickness. A transected breech after Briley lengthening a short cone
4. The difficulty sourcing 2 1/2" shells for the non-reloader is unlikely to resolve any time soon.
Important declaimer: there is NO DATA regarding the pressure rise when using long for chamber length modern plastic hull shells in 16g or 20g.