That is a common misconception. The progressive burning powder loads that came out in the 1920s, Western Cartridge Co.'s Super-X load leading the way in 1922, were not higher pressure than the heaviest old bulk and dense smokeless powder loads which had been offered from the late 1890s up to that time. In the 12-gauge 2 3/4 inch and longer shells, 3 1/2 drams bulk smokeless pushing 1 1/4 ounces of shot or 28-grains of dense smokeless powder such as Infallible or Ballistite pushing that same 1 1/4 ounces of shot. What the new progressive burning smokeless powders did was push an equal shot charge at a higher velocity or a heavier shot charge at an equal velocity.
I've now scored some old DuPont Smokeless Shotgun Powders books from the late 1920s and early 1930s. These booklets are very lengthy tomes, 96 and 103 pages, promoting their DuPont Oval progressive burning powder, which allowed the development of the high velocity loads like Western's Super-X, Remington's Nitro Express, etc. In the manual they show a table giving the pressure of the 3 1/2 drams DuPont Bulk Smokeless pushing 1 1/4 ounces of #6 shot as 11,700 pounds, with 3 1/2 drams of Schultze 11,800 pounds, and the same load pushed by 28-grains of Ballistite at 12,600 pounds!! Velocities were all given over 40 yards, not the muzzle velocity figures we are use to seeing today. The DuPont bulk gave 943 fps, the Schultze gave 941 fps, and the Ballistite 966 fps. Meanwhile 40 grains of their DuPont Oval gave that 1 1/4 ounce of #6 shot a velocity of 981 fps with a pressure of only 9,400 pounds.
I note that in my Western Cartridge Co. catalogues that up through their April 1, 1930, catalogue they are still giveing drams bulk and grains dense in their loads in the Field and Record shells. By the March 1, 1931, they had introduced non-corrosive priming in their shotshells and the loads in their Xpert, Field and Record shells are given in drams equiv. Western Cartridge Co. never gave drams, grains or drams equiv. for their Super-X loads. Just said they were a Super Excellant Long Range Load --
Here from the mid- to late 1930s the essentially the same Winchester loads give a dram equiv. and the Westerns say "Maximum Load" --
