FWIW, My father was a licensed commercial ammunition reloader and he used ground walnut shells to clean fired brass (in a cement tumbler!).
I reload my own ammunition here. I use ground corn cobs and walnut shells in a vibratory polisher to clean and polish empty cases. These both do not erode the base material and leave a beautiful polished finish on the cases. I also use a very good hand cleaner which contains ground walnut shells. Easy on the hands and cleans like crazy. There are all sorts of less aggressive cleaning methods out there.
