The development of progressive burning shotgun propellants however in the 1920's allowed heavier loads to be used with no increase in max pressure
There was an increase in pressure when we transitioned to the then-new 2 3/4" loads like the Super-X during the 1920's. SAAMI even had different proof and service pressure limits for both. As reported by LTC Calvin Goddard, "Army Ordnance" magazine, 1934: 2 5/8" 12ga, mean proof pressure 13,700 psi, service pressure 9500 psi. 2 3/4" 12ga, mean proof pressure 15,900 psi, service pressure 10,500 psi. Given the date of that information, we need to interpret psi as LUP, since it would have been measured with lead crushers .
