Slug the bore. My Buchel was A 318? Cant remember now,sold the gun.  Made a special sizing die at 320. Also used 40-1 worked best with a 200 gr bullet. but every gun different. Good luck in your developing load.....

I have had many of these through the years and the groove diameters have been anywhere between .316 and .323. I currently have 2 rifles in that caliber but one is a German stalking rifle and the other is a 71 Mauser that has the barrel from an Aydt offhand rifle. Both of these do like the stop ring bullets from Buffalo Arms moulds they make in .316 and .319. I've never had a rifle that required a case longer than 46mm and I've had some German ammo that had only a 45mm length. The main thing is to make sure that the chamber has not been modified after arriving here. I believe many brought home from the war ones were rechambered with a 32-40 reamer or even attempted with a 32 Win Spcl one. I'll have to go back through my records and see what has worked well through the years. You're on the right track with 32-40 type loads and I seem to recall some from 60 years ago that even liked some 2400.

I have 13 German schuetzen rifles, 3 stalking rifles. 8 of the Schuetzen rifles are 8.15x46r, 3 are 22lr, 1 was rechambered to 303 and 1 to 32 Winchester Special. All of the 8.15x46r slug less than .320. I shoot a 175gr stop ring bullet or a spire point 190gr stop ring bullet. I shoot 12-14gr IMR4227, Remington 9 1/2 LR primers. I have never shot fixed ammo in the schuetzen rifles. If your rifle is late 19th or early 20th century, the bore size may be given in gauge not millimeters.
Mike