It has no markings that I can find. Nothing to indicate caliber. Seems to be in good condition. I would love to know more about it, but can not seem to find anything that looks quite like this via Google searches.
I believe that's called a "traveling pistol" from a long time ago, and if I remember correctly they were to stop highwaymen. I remember seeing a picture of one in a book I had when I was a kid. Pretty cool!