Looks to me like the author and editors spent more time covering themselves in legaleze than imparting any useful information.
I don't believe that was originally a McIntosh quote, but from a letter Lou Smith sent to a customer inquiring about one of Ithaca's old Damascus barrel doubles. From the days when the industry was perpetuating that Damascus & Twist barrel myth to try and sell new guns in the Great Depression.
FWIW, the inside flap of every box of RST ammo I have from their lightest 5/8 ounce 2 1/2 inch 28-gauge load to the heaviest 12-gauge 2 3/4 inch Pigeon Loads carries the warning "To prevent injury to shooters or bystanders use only in modern shotguns (not in Damascus twist barrels, etc.)...." The lawyers carry the day!!
From the late 1890s to well after The Great War, the heaviest 12-gauge bulk or dense smokeless powder loads our North American ammunition manufacturers offered were 1 1/4 ounce of shot pushed by 3 1/2 drams of bulk smokeless powder, such as DuPont, E.C., Schultze, Dead Shot, etc. --
or that same 1 1/4 ounce of shot being pushed by 28-grains of dense smokeless powder, such as Infallible or Ballistite --
offered in 2 3/4 inch and longer paper hulls. A couple of DuPont
Smokeless Shotgun Powders booklets in my collection from the late 1920s and early 1930s state that the pressures of these loads were in the 11700 to 12600 pounds range.
