the british double rifle dont fall from the sky. there was an evolution between 1860 and lets say 1885 wenn all was ready only missed the nitro powder and modern bullets.
it all startet with the high quality muzzle loader double rifle from the hands of joseph manton which was on his peak in 1860. they had to find a breech loading action and a cartridge, put an unusual high black powder load in it and the black powder express rifle was born. all in the years when military trials for a new military cartridge were held with 450, 500 and 577 caliber cases of different length and the pinfire action and cartridge reach britain for showing that the hole concept works.

I would say this rifle here is chamber for the 24 ga 2,5" pinfire paper shell, not realy a express round but they had learned from this