Robert Hubner was a Master Gunmaker in Darmstadt, Germany - and is today a retailer:
http://maps.google.com/maps/place?cl...61334772983163
Your Drilling has short 16ga chambers, as noted above - which, although will accept an UNFIRED 2-3/4" shell, are too short for the FIRED shell's length, as the longer shell's crimp will obstruct the passage of the shot charge ILO lying down in the chamber mouth, out of the way.
The rifle bbl is chambered for the 9.3x72R cartridge - "8.8" is the bore, and not the groove, diameter in millimeters.
It's intertesting, if not curious, to view a short-chambered, hammergun (ca. pre-WWI features) with proof, view & inspection marks that were not used until after 1950.
Hubner bought actions "in the white" from various makers, and completed the guns in his small shop.
IMO, excepting it's rounded action (The Sauer), your drilling looks like this blackpowder Sauer hammergun, down to the island back-action locks:
A few better, bright & clear pics of the marking & stampings on the barrel bottoms, and action insides (if any) might produce/ID a maker's mark.
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