tinker, every gun shooting the paper case was able to do it with brass! the stop ring diameter is what counts. I believe he was a little bit oversized for fitting every rifle.Its clear that a brass case have a bigger inside diameter than a paper cases. RWS know's it also, make a paper case bullet 17,10 mm in dia. and a brass case bullet 18,00mm. its only for the correct fit in the cases.
such bore rifles show a big difference in bore/groove dimensions.

thats your way:"Tinker, run pure balls of a diameter slightly over groove diameter"
but look also for the inside diameter of your case