Here you go Gil. "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" by Wallace H Coxe, 1931.
Ballistite is an early Dense Smokeless - higher chamber pressure
Schultze and DuPont Bulk Smokeless
DuPont Oval an early progressive burning powder - lower chamber but slightly higher at 4"
DuPont MX Smokeless was a Dense Multi Base Powder introduced about 1930. 25.5 grains was a 3 Dram Eq. and it was promoted as a 1 1/4 oz. Trap load. It was replaced in the 1954 with IMR PB.
This shows how pressure is higher at 18" with Dense vs. Bulk Smokeless
32 Gr Ballistite (1 1/8 oz. at 1232 fps = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624 psi)
2 1/2" - 2.2 Tons = 6272 psi
6” - 1.2 Tons = 3718 psi
12” - .81 Tons = 1602 psi
18” - .33 Tons
24” - .21 Tons
42 Gr Schultze 1 1/8 oz. at 1220
Breech - 1.69 Tons = 4558 psi
2 1/2” - 1.49 Tons = 3886 psi
6” - 1.36 Tons = 3450 psi
12” - .92 Tons = 1971 psi
18” - .55 Tons = 1178 psi
24” - .32 Tons
This is a pressure - time curve from Neil Winston -
www.claytargettesting.com. The shot leaves the (30") barrel at about 3.5 milliseconds after ignition.
The pressure is down to about 500 psi by 2 milliseconds.
More here
More here
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/preview 
