Parker Bros. guns were seriously over-bore back in the 1880s and 1890s. Chamberlin Cartridge Co. loaded special cartridges for Parker Bros. guns with 11-gauge wads in the 12-gauge shells or 9-gauge wads in the 10-gauge shells.
1886-7 page 6 Parker Load Highlighted.jpg
My 1889 Quality GH 12-gauge had bores that measure .757-inch.