Promo

thanks for showing us this interesting gun, and in high-quality pictures, too!
Really interesting, a paradox gun with all the details of a good rifle: set trigger, diopter.

My guess is this is a 12 gauge made for use with brass shells: outside dimensions same as a regular 12 gauge, but case walls much thinner than of a paper hull, necessitating a larger ball (or bullet) and a larger bore
see this excerpt of a 1904 DWM catalog with case dimensions

Regards
fuhrmann