Your rifle seems to be constructed to allow it to be loaded near the muzzle of the barrel, which is typical of a bolt action Zimmerstutzen or "parlor rifle" as it is usually translated. Your 4mm ammunition is not powerful enough to make it down a full length barrel, although the full length barrel is needed for sighting purposes.
Zimmerstutzen were made to be fired indoors into a bullet trap holding special targets adjusted for indoor use. The range is 15 meters and the ten ring of the target measures 4.5mm. Each of the smaller value rings is also 4.5mm in width and rings 10-6 are included in the black bullseye of the target. The rest of the rings are in the white portion of the target and are numbered down to "1".
The targets themselves are 85.5 mm long and 40mm wide.
Zimmerstutzen have been largely replaced by precision air rifles.
