I use large Medium and fine tooth mill files. I like large files, you can keep them flatter, as you pull them. You'll need to chalk them, to keep chips from inbedding in the teeth, and galding. A wire brush is needed to keep the teeth clean. As you draw the file toward you, the metal will roll out like fine hair. If the barrel is pre-fitted to the stock, I draw file the top flat, the next two down, and not too much on the side flats. On a long rifle, I don't mess with the bottom three flats. I finish out with Emory cloth. Emory cloth is a lot tuffer than sand paper, and works better on metal.
You can get a quality bastard mill file, anywhere quality tools are sold. 10" Nichoson smooth mill file, will be all you need. Don't throw it down, treat it like a precision tool.
