To me, it appears to be a muzzleloader - of the pepper-box revolver type. The lever on the side is the release for turning the barrels to bring them to bear under the hammer.

It has been dry fired so much, the nipples have been beaten flat.

This type of gun was very popular in the States, for a while. Not very accurate and oft times of small calibre, but examples up to .45 are found.
Some of them rotated with the pulling of the trigger while others had to be turned manually, as-is your example, Casper.

1st search found this. Here's a page of different ones.

https://www.google.com/search?q=pepper+b...i5z9nqJoNSvgQM: