You show a Flobert pistol, one of many many inexpensive small bores made in Belgium in late 19- early 20th century.
Nominal caliber is probably 6mm CB, close enough to .22 CB Caps
Yours is the “Remington” pattern, so called because it operates like a Remington Rolling Block BUT that doesn’t mean it is strong. CB caps are about all it is good for.
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