You probably have a Westley Richards English shotgun, IIRC made in Birmingham. I once had to pass up a great deal on a WR drilling, a double with one barrel in 12 guage, the other in 11mm rifle. The rifle cartridge was about 4-1/2 inches long. Made for hunting birds when big scratchey critters might show up. It came in the shop for extractor repair. Yours of course is percussion and of older vintage, very desirable in my eyes. Good find!