It looks like the remnants of a wall gun to me, about .71 caliber, with the barrel heavily cut back, perhaps losing 50-60% of original length. Must have been cut at both ends, based on current location of the rear sight almost at the breech. Would like to know where the barrel is signed by Miksh. Barrel makers usually marked their barrels underneath, or on an oblique flat near the breech. Gunmakers usually marked their barrels on the top barrel flat...for easy reading and "advertising." If cut heavily at breech as it appears, I would think any barrel maker's mark would be lost...and perhaps any maker's mark. The crude stocking could well have been done by someone who worked primarily as a blacksmith.
What really puzzles me is why anyone would shorten a wall gun to this extent. It's still too heavy, too cumbersome, and too large a bore, for most later applications where old rifles were cut down to slaughter stock, etc., and are often referred to as "hog rifles." Most of the [few] wall guns I've seen are very plain guns, a basic tool for protection in a specific situation, otherwise with very limited usefulness. Shelby Gallien