I do as Mike does, just file it with a sharp triangular file by eye. If you have a copy of RCA, or something similar, take a look at the originals. Most look like they were done by hand in just a few minutes. When they built the original guns, time was money and you will see lots of minor imperfections. That does not mean sloppy work, it just means careful hand work without the use of gauges or machine tools which we are used to. To me a longrifle made with modifications like you propose done by hand and eye, look far superior and "warmer" than ultra-precise, exactly measured features that looked like they were done on a CNC machine.