My model carronade has quite a few very small metal (mostly brass) components. They are fiddly and a bit difficult to hold while finishing (filing and sanding and polishing).
So I bought a tumbler which is designed for polishing metal jewelry and gemstones, and gave it a go.
It cost just under $300, including a kilo of stainless steel bits, and some polishing compound. It is designed to run for weeks at a time when polishing rocks, but I find that 30-90 minutes is enough for my brass parts.
I am still experimenting with the tumbler. So far I have used only the stainless steel shapes to do the polishing. I will try some abrasive compounds soon. Garnet dust seems to be the commonest abrasive.