Hmm, if you can see a rough spot, then you can grind it down very carefully and slowly. Best thing to use is a Dremel with one of those long, bendable neck-extensions and a small grinder on the end. If you can't get your hands on one of those, try a rat file, or similar thin file. After you are done filing, make sure that there are no filings or dust left in the mag. Good luck!
Hueco