I purchased two rock tumblers a couple of months ago. Within a day the larger one had stopped working so I was left to experiment with the smaller one. Well, it did the job somewhat, but not fully. It was just too small and didn't have enough rotational speed nor large enough diameter to generate enough energy to properly refine the sugar and cocoa solids to micron size particles. The chocolate was smoother, but not smooth enough. You could tell it was still homemade. The new replacement tumbler showed up earlier this week so I set it up to refine some sugar. I put the about three cups of sugar and three cups of SS shot in a 2 quart glass jar. I had to put some larger diameter tubing on the rollers so the glass container would rise above the sides of the tumbler and not scrape the sides, but this had the added benefit of increasing the rotational speed of the drum. You can really see the sugar and shot moving well.
Oh, and someone asked about the glass. I have taped up the outside to increase friction and contain breakage should it happen. And no, I am not terribly worried about "grinding" the inside of the glass surface. Both the sugar and chocolate seem to create there own protective coating pretty quickly, so no glass bit show up in either.
I will report back how this new set up works.