I came across this rifle the other day. Very unsure of what it is. Left side of the barrel is stamped with "Dachau" and the right side is "A. Kliegel" I know this is not and airgun forum and im thinking thats what it may be. It doesn't seem to break open as a single shot would, and there is a trap door out on the last 10" of barrel to load something in it there. The barrel size seems to be about .15 caliber. as a .177 bb will not fit down it. The trigger guard pulls down as if to cock something, and the set trigger is functional as well
The reason for that breech system was simple. The little 4mm round was originally made for pistols; it had no powder and the primer was not enough to reliably get the pellet out of a long barrel rifle.