I have not polished a firearm, yet... I have polished many other things from aluminum, stainless, magnesium and brass parts for my VW and sportbike hobbies. I have been told to use 400, 800 and 1200 grit wet sandpaper and final polish with Mothers mag polish. I was not happy to end up with a "cloudy" mirror finish. IMO for best results, one has to start with a wet sanding paper rough enough to get rid of the tool marks and eventually using the finer and finer grit of wet sandpaper. I may start with the 320, then work the 400, 600, 800, 1000, 1200, 1500 then stop at 2000 grit wet sandpaper. Then off to the bench model buffer. I start buffing with the red rouge, clean the buffers and finally buff with the white rouge. Then for the final polish with Never-Dull. In the end one ends up with a mirror like finish.
In the past, I have always wanted to stop wet sanding once I hit the 400 grit and stick with the brushed look. I have also stopped and considered media blasting. But in the end, I usually suck it up and go through with it.
I would think, doing a firearm will be similar. A Dremel would work, or pneumatic grinders with various felt bobs and various buffing compounds. In the end it is all up to the one doing the job. What does one consider polished? Is shiny enough or does one want to see him/herself in the object in hand?