A gunmaker named Richard Casleton (I've been told he's very particular about spelling his last name without a 't') is Head of Gunmaking and one of the founding partners of Joseph Manton London (www.josephmantonlondon.com). I think this is the person you want. Try writing or phoning Ian Spencer http://www.josephmantonlondon.com/contact-us/ - he's an exceptionally nice person and I'm sure will set you on the right track.

Russian Hunting Shotguns