I just got through doing the same thing you are trying to do with a 10.75x60r. Mine has turned out to be some sort of propitiatory cartridge. I have been able to make brass and I have made a die to bump up a jacketed bullet to the proper diameter. I am still working on the regulation. German rifles tend to be all over the board on bore size so a proper bore slug will be necessary. As I understand it there bores started to become standardized after the 1920's. The German Gun collectors forum should be able to provide some usefull information. On the sides or the top of your barrels should be a marking telling you what material your bullet is constructed of along with the weight in grams. There should also be a nitro proof mark. The material the bullet is made out of and the weight will help when it comes to figuring out reloading. I also used it to check the rifling twist rate. I used one of the twist rate calculators on line to see what the proper twist for the projectile and physically checked the twist in the barrels. All of this sounds a bit mundane but it will become important if you cant match your measurements to any specific cartridge.