The rifle barrel of your drilling was rifled by Wilhelm Kelber, who was a specialist barrel borer and rifler. His WK stamp is found on many rifle barrels on rifles by Schüler, Krieghoff, Sauer &Sohn and others. The proofmarks, 7.8mm bore diameter, 72mm case length, 12gramm bullet, clearly show it to be chambered for the 8x72 R. The photo shows such a cartridge besides a 9.3x72R, flanked by two tenpacks, front and back.
The 8x72R is often erroneously called a Sauer & Sohn cartridge, likely because of the tapered shape, similar to the 8x58R S&S. In fact it was the first cartridge developed by Wilhelm Brenneke, 1898. Brenneke either tried to improve the performance of the S&S number, or he tried to put the performance of the 8x57IR into a slimmer package, more suitable for the then standard slim drilling frames. The cartridge is out of print for half a century by now, so you have to rely on handloading. First, find a die set and 9.3x72R brass. Taper the cases down in the FL die. Load with "I", .318" diameter bullets. Use weak American level 8mm Mauser data.
The rifle barrel is made of steel by Krupp, the shot tubes of steel by Röchling.
In the 1920-30s Heinrich Krieghoff owned two separate companies, that is "Sempert & Krieghoff", making hunting arms, mostly drillings, and "Heinrich Krieghoff", machining parts like drilling frames for the trade and later making military arms like the famous "Krieghoff Lugers".
Note that the inscription on the frame does not read Sempert, but Semper Supra, Latin for "ever superior". Both words were used as trademarks by Krieghoff to this day, f.i. the once popular insert barrels to shoot .22 lr or magnum or Hornet cartridges from shot tubes were called "Semper" by Krieghoff in the 1960s.
