To me it looks like the frame and barrel of a muzzleloading percussion underhammer pistol. Do an image search on "underhammmer" and you will see what I mean. It seems to have a front sight, the trigger is pinned to the frame in the proper manner, and i believe the extra piece of metal inside the frame is the remains of the mainspring. The notch on the inside of the frame is where the end of the spring (now missing) would have rested. There seems to be a small notch on the underside of the breech end of the barrel. Is there any evidence of a hole there? If so, this is where the percussion nipple would have been screwed in. Most underhammer pistols were made in New England in the 1830-1860 era, but the relative simplicity of their mechanisms encouraged tinkerers to make them at home in all parts of the country.