I suspect you have a single shot, German Smoothbore, for a 6mm type Rimfire?
May be Bolt, lifting Block, Rolling Block, or Break open Gun...wide variety.
Ammo to use: .22 Rat Shot, 6mm Flobert Shot ( paper case) 5,7 Flobert.
There are several European Brands of Flobert cartridges available, but most US users settle for Brass cased .22RF Shot.
6mm Glatt is the usual indication for a .22 cal. SMOOTH Bore ( " GartenFlinte").
If it was Rifled, it would be a " GartenBuchse"
Fun Gun if in good working condition ( general Garden Pests--- Rats, Mice, small Birds, etc)
DocAV