(Thomas K.) Bacon & Co. and W.W. Marston made guns just like that, and both used the name Union Arms Co., but my source (Flayderman's) doesn't indicate a connection between the two manufacturers. Stocking & Co. also made a very similar pistol; again I can find no connection with the other two. While they look like screw [off] barrels, they were not; the octagonal part was decoration, not a place for a wrench.
All three of those makers also made pepperboxes, and the center hammer was derived from that of the pepperbox.
As to value, it would probably retail at around $150-200 in that condition.
(I don't know what an old barn that is still in working condition is worth.