Under the 1925-1954 Rules of Proof, the 2 3/4 12g max. service load was 3 3/8 Dr. Eq. with 1 1/4 oz. shot, with a mean pressure of 3 1/2 tons = 10,640 psi by Burrards conversion. (Primarily for heavier Waterfowl guns)
Major Sir Gerald Burrard, The Modern Shotgun, Volume II, The Cartridge, 3rd Revised Edition in discussing shotshell pressures for 12g 2 3/4 1 1/4 oz. 3 1/4 Dr. Eq. stated "Standard Service" 9,296 psi; "Maximum Service" 11,984 psi.
The 1954 Rules of Proof Highest Mean Service Pressure equivalent transducer values as converted from LUP by Burrards formula 3 1/4 tons = 9,800 psi = 676 BAR. "Standard Service Pressure" for most loads however was about 8000 psi per John Brindle in The Double Gun Journal, Black Powder & Smokeless, Damascus & Steel; Volume 5, Issue 3, 1994, Some Modern Fallacies Part 5, p. 11.
The CIP transducer Maximal Average (Service) Pressure" for 2 3/4" chamber or 3 1/4 tons is 780 BAR = 11,313 PSI
