[Helleri]

It's a Swivel Gun (by the looks of it 18th c. french naval if authentic). A small caliber anti-personnel cannon (sometimes musket) primarily for warships and merchant vessels. An old bronze one like that would have originally been mounted to a U shaped yoke via those trunions. This would allow it to pivot on the vertical axis. And the long round rod coming off the bottom of that yoke would be dropped into a hole in a ships rails or Bulwarks (sometimes even on crows nests). This would allow it to pivot on the horizontal axis. That knob on the back. That's what you'd toggle and stabilize it by (yeah...Unfortunately with your hand on the early ones).

These would mostly be positioned fore an aft on a ship, in case of close pursuit or a crossing the T maneuver. If the sailor manning it was on his game and a little lucky. He'd have a good chance at taking out a crucial member of an enemy crew.

You might actually want to make sure it isn't still loaded. They were generally kept loaded. As when it was time to put it into action. there wasn't time to load or reload it considering the very specific circumstances of it's use. You had one shot pretty much. And you had to try to make it count. That could be an old fuse in the touch hole and not corrosion. It would be flush fused to insure getting a quick shot off.