That is a nice little rifle, but you really need to be sure what cartridge it is chambered for before you shoot it. I don't see an N for nitro in the original proofs, so it would lead me to believe it was originally for black powder. I dont see theSt. M G for jacketed bullets either. I've had guns of that time period that were so marked and i believe they would be there if proved for them. But German proofs are a study unto themselves and i am not an expert. As others have said, you need a chamber cast. There are a lot of 9.3 cartridges in that era. There are several 9.3x72's, a 9.3x70 and the 9.3x74 black powder, nitro and one that was only for lead and Some other 9.3's that I cantrecall. Some are very similar in dimensions but very different in loading and pressures. Some of the x72's are not interchangeable. To make things worse, there were some rechambered to cartridges the actions were not designed to handle after they came to the states. After you do a chamber cast, you will need to slug the bore, best done at three places, just in front of the chamber, a full length one and one just the last couple inches at the muzzle. Then put the pieces of the puzzle together. Cartridges of the World is a start, but after you get all your dimensions, a visit to the German Gun Collectors forum would be a good idea. There are very knowledgeable folks there that will willingly help you. Then you should have a pretty good feel for the correct cartridge. Wish you well on an interesting project.
Bfly