I hate to ask this, but are we sure that this is American-made? The half-stock with the thimble soldered directly to the barrel like that is a common Germanic style, and the cheekpiece and carving make me think of Scandinavia. Lock looks Germanic to me, too, as do those very prominent screw heads on the buttplate.
They occasionally did build shotguns out of recycled parts in Europe - I've got a German percussion c. 1840 with a converted c. 1750 lock (and I suspect the matching barrel). This strikes me as something of the same kidney. I'm inclined to think that it is a late 18th century/Napoleonic era musket restocked as an inexpensive shotgun, very likely in Scandinavia (which did have a hunting culture that included poorer folks, not just the wealthy landowners).