Forgotten Weapons found a sporting Mauser for auction at Rock Island Arsenal and at first it seems to be nothing special... except that it has an integral .22 barrel under the 7x57mm barrel.
Manufactured by Christoph Funk of Suhl, here's the Forgotten Weapons page
http://www.forgottenweapons.com/clev...-rifle-at-ria/
and a pdf of general info but including some more about the double rifle
http://www.cornellpubs.com/downloads/Funk-Suhl.pdf