I'd be guessing, but it looks like a Flobert saloon gun to me. It shoots a special CB cap cartridge, and was meant to be shot indorrs [really].
This pastime was a popular post-prandial leisure occupation in Europe in the mid to late 19th century - Flobert, the inventor of the system, patented a wide range of very-low powered cartridges from 4.5 to 9mm specially for this purpose.
DO not try anything else other than the correct, low-powered BP Flobert cartridges in this old gun.
tac