You can easily use steel scrapers on brass as well as wood. Also hardened pieces of burnishing rod work well on brass. Additionally, gritpaper was available although it's never been clear exactly how widespread it may have been used. Loose pumice, rottenstone and assorted other abrasives were likewise advertised and available and can easily be used with scrap leather or softwood backing with a little oil.
Very fine files followed by just a bit of abrasive polish afterwards imho were most likely the most common finish on brass mounted rifles.
