That is what the Winchester ammo catalog on ebay shows. The heaviest black powder load offered is 7 drams pushing 2 1/4-ounce of shot.
March 1, 1906 05.jpg
Union Metallic Cartridge Co. showed bulk smokeless loads as heavy as the black powder loads in 1900 --
8-gauge TRAP Shells 1900.png
but backed off the smokeless loads for 1901 --
8-gauge TRAP Shells 1901.png