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We are fortunate today to have an abundance of turn-of-the-century pressure data, which can be found here
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/edit

I stumbled upon another interesting source; “Shotgun Ballistics”, Charles Askins, Outdoor Life, July 1917
https://archive.org/details/sim_outdoor-life_1917-07_40_1/page/76/mode/2up

Pressure X 2240 converted to PSI (not Burrard's formula and I am confident that is how DuPont did it), which requires adding 10-14% for modern piezo transducer numbers.

Note modern SAAMI standards allow +/- 90 FPS and +/- 900 PSI = .4 Long Tons

12g 2 3/4” 1 1/8 oz.
……...…………...3 Dram…..3 1/4…..3 1/2…..3 3/4…....4
DuPont Bulk…...7,750……7,840….8,019…..9,744….10,640 PSI
“Schultz” Bulk…8,064……8,960….9,520…..9,878….9,744 PSI



16g 2 7/8” 1 oz.
………...……….2 1/2 Dram…..2 3/4…....3…....….3 1/8..…..3 1/4
DuPont Bulk….7,683…….....8,243..…9,223......9,811…..10,080 PSI
“Schultze”…….8,333…………9,677..…10,461..…12,790..…11,200 PSI



20g 2 7/8” 7/8 oz.
………….…...…..2 1/4 Dram…2 1/2…....2 3/4…..3
DuPont Bulk……10,685…..…12,208..…12,902…14,538 PSI
“Schultze”………11,715...…12,499…..12,858…13,530 PSI

Note the maximum pressure observed for the standard 7/8 oz. 2 1/2 Dram “Schultze” load was 13,978 PSI + 10-14% is approaching proof load pressure at > 15,500 PSI