[jakogut]

Indeed, it looks to be a crude double barrel shotgun loaded with black powder. Such firearms are easier to make, as black powder has lower peak pressures which don't necessitate a very sturdy construction, especially in large bores. Also, black powder can be DIY manufactured from commonly available supplies like charcoal, sulfur, and potassium nitrate, unlike modern smokeless powders, which have a much more complex manufacturing process.