Definitely a bullet barrel. Appears to be d/t for mounting Unertl/Fecker type scope blocks too. Might have been mounted to some sort of underhammer action or used on a muzzleloading breechloader. There are such things. A friend has one. False muzzle for loading a conical from the front, then loading a primed and powdered case from the breech. Interesting rifles.

A friend of mine shoots several varieties of single-shot breechloading in competition. He shoots fixed ammo, muzzleloading, muzzle/breechloading, and breech seating. Those guys will shoot an entire 200 round match with one case that's de-reprimed at the line, reloaded with powder and wad, then the bullet carefully and precisely seated in the chamber, then the case loaded behind. It makes traditional muzzleloading look positively speedy by comparison. Accuracy is amazing. I too know better than to get involved--at least so far.