I recently purchased an underlever German stalking rifle from 1902. It was made in Suhl and has all the German proof marks except the nitro proof because of when it was made. It is chambered in 8.15×46R. It has double set triggers and is a beautiful gun. Im curious if I used a slow burning powder in light loads, could I use smokeless powder on this rifle?
Cheers

I have a couple of black-powder cartridge rifles that I shoot with smokeless.
Both have slightly more pronounced bottle-necks that yours (for reference, .577-450 and .450 Westley Richards No.2 Musket), and the cases I use are "reduced volume". In other words, they are lathe-turned, with the outsides conforming to the rifle's chamber but the inside is just a straight, drilled hole slightly smaller than bullet diameter, with the neck section obviously bored out to bullet diameter, thus forming a step to stop the bullet.

Reloading requires no dies. It's simply a matter of punching out the primer, seating a new one, pouring in the powder, placing a wad (cut from the polystyrene tubs some fresh produce is sold in here), then pushing in the bullet (basically just a finger-tight fit, they are effectively held in place by the bullet lube).

However, the powder I use is not an especially slow powder. I use Somchem S335, typically used for calibers like .223.
I get basically the old factory ballistics without any signs of issues. Have never tried harder.

The process was fairly common here a few decades back as black powder was considerably harder to source and use (all sorts of permits were required to own it and to transport it).