Imported into the US through the appropriately-named Navy Arms company, this is a Greener Martini action built into a "Light Harpoon Gun" by Webley & Scott in the UK. These were built as legitimate hunting arms, although they are far better known today for the appearance of one in the movie Jaws.
The intended use of the gun is to shoot large fish from an overhang on the bow of a fishing boat. It has an effective range of about 30 meters, and the line attached to the harpoon can handle a 300 pound load (with stronger lines available up to 1200 pound). A blank .38 Special cartridge is used to fire, and the harpoon has two large folding hooks to anchor it in a fish after striking.
The gun was shipped in a case complete with everything needed for use, including the gun, three harpoons, six lines, and two line frames. I was somewhat surprised to see that in Jaws, the gun is actually assembled and used properly, with the allowance that the line frame was not necessary. Well done, Quint!
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