Fine files, oil, and fine grit paper/cloth mixed with some elbow grease. I’ve seen stones used for other types of work, but not for polishing up internals to a final finish, at least not by the guys I’ve seen work. I personally only use files and cloth. I like to polish internals up to about a “chrome” appearance. It definitely aids in corrosion control and smooths out how the action works. If a piece has alot of tooling or file marks on it, I’ll draw them out using fine files. If the parts being polished up have to maintain some critical geometry (certain trigger parts), obviously you only do what you can to clean up the surfaces and not change any of the final fit unless you absolutely need to. I was taught to polish in the direction the parts will move too, I believe that’s fairly critical in maintaining that fine, hand finished appearance.