Now I've only filed out a few bbls, but I have to mention that I don't find a benefit to chalking. Matter o' fact, I tried it some and got worse pinning that without. I keep the file clean enough and just don't have a problem that chalking helps. It does help with soft metals, but I don't use it on steel.
Different (file) strokes I guess. Also, I push the file. I learned on a surplus Swedish gun of later mfg. BP bbls are longer but easier and I really like Bob's implications of shorter file= tighter cut. Any time you can use less sanding paper, the better in my book.
Draw filing metal is a bit like planing wood. Enjoy, I do.