This is a double barrel muzzleloading derringer. No name brand on the gun anywhere. Just a nicely stamped serial number in the butt of the frame. It is very heavy. Gun looks to be machined out of brass, or possibly bronze?? The barrel is rifled, and mics out at around .372. Very well made, and the hammers also have half cock position for safety. Looks to take a #11 cap for ignition. Grips are plastic. Please look at the pics in the link, and any information would be greatly appreciated.
http://www.ranchoweb.com/folder/45831

Standard black powder calibers -- as I understand it -- are .31 and .44.

Did you take the grips off and look for marks?

It's brass. Bronze has a tinny/grey color. It's a replica, but I'm not sure what it replicates. I've seen the design in antique derringers, but have no clue who made them.

Spain and Italy make a lot of black powder guns. In the lesser brands, they don't mark them much.