This is a very interesting item from my mezurashii 珍しいもの (unusual items) collection. When fully assembled, it appears to be a functional hand cannon. However, when you remove the end of the muzzle, a writing brush slides out. When you extract the tamping rod, you see that it is a small paper knife. Finally, when you pull the trigger, an inkwell in the stock is exposed.
All in all, a very interesting item.