Harpoon gun? Line throwing gun? We're really just shooting in the dark here..
I didn't see any kind of conventional attachment points for a stock. Either I missed them or it is something never made to use a regular stock.
If not, that argues against it being a rifle type line throwing or harpoon gun. Could be something meant to be mounted in a cradle or pintle, but again, nothing I saw shows clear attachment points (screw holes, or lugs or...??)
A bolt handle means something is meant to be opened and closed. Beyond that, we'd need a better view of the "breech" mechanism.
I feel pretty sure there are parts missing, which makes it more difficult to ID it as to type/use.
I once saw a single shot .50 "rifle" made from a Browning M2HB barrel. It was the full, unaltered machine gun barrel, with a homemade breech block screwed on the back end. The barrel was drilled to take pins (maybe screws) on each side, fitted into a U bracket (like an oarlock) mounted on a tripod. Barrel was also drilled and tapped for scope blocks and had a scope mounted.
The home made breech block had a firing pin and spring, a safety, and a trigger that stuck down from it. No trigger guard. Mechanically, it would have worked. Though I strongly suspect the gun was intended to have the gunner sight it, then get out of the way before pulling the trigger!
A good pic of the breech mechanism, and chamber, if possible would give us more to guess about. Right now, I am wondering if it was made for an actual cartridge, or for something else, like a bag charge, perhaps. Just wild guesses at this point, sorry. Doesn't match (or come close) to any commercial firearm I am aware of, I still think it might be a homemade "something", but unsure just what...
