Is this a percussion gun, a flintlock, or a flintlock converted to percussion? The percussion cap was invented around 1820. My books also say Collis was active around 1800, but no more info; you might be able to nail it down a little closer if you take the barrels off and post a picture of the proofs (there should be a cross-key in the forearm that you can tap out in one direction, then lift the barrels out of the stock, to get a view of the undersides where they would be proofed. I'd also make sure that this shotgun isn't loaded, since many of these muzzleloaders were loaded up once upon a time in preparation for immediate use, then put away and forgotten about; you can use a ramrod down the barrel to check, and if that rod doesn't go all the way to the nipples/vent holes, treat it as loaded.
Gun control in Canada: making the streets safer for rapists, muggers, and other violent criminals since 1936.