It is also well documented that turn-of-the-century shells generated the same or even greater pressures than today's standard loadshttps://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/preview
ed - if you have evidence to the contrary please post it.
And from a comment in Sporting Guns and Gunpowders
regarding an additional study after the 1891 Proof House Trial Report, and published in The Field
June 6, 1891 by Horatio F. Phillips, a “staff experimenter” with The Field
. This is in reference to the standard tubes used in the Trial which had no chambers cut and a greater wall thickness than finished barrels
“These experiments serve to show what a very large margin of strength there is in a good gun barrel, when ordinary charges are used. The (Damascus) barrels which gave way earliest…had withstood the strains of…about four times
as great as the regulation proof; while the steel barrels (Siemens-Martin and English “Superior Barrel Steel”) were tested…with charges averaging nearly five times
as much as the ordinary proof-charge.”
It has also been well documented that the "rough forged tubes" were first proved in Belgium, and the finished guns were proved by the U.S. makers in-house.
Belgian First Obligatory Proof Load for “Double-Barreled Breech-Loading Sporting Guns” 12g breech plugged tubes was 11.8 Drams powder and 1.12 oz. shot
Ithaca advertisements stated that barrels were proved with a “double charge of powder and 1 1/2 times the normal shot load”; or (possibly) 6 1/2 Drams Black Powder with 1 2/3 oz. of shot if the standard load was 1 1/8 oz. shot and 3 1/4 Dram Eq. Bulk Smokeless.