what I did to fit my new model 41 barrel to my 1959 vintage model 41. Preface.... I'm not a gunsmith, just an old former toolmaker. This may not work for you, but it did work well for me. If you take off too much material, you're gonna be in a bind...... don't holler at me. .... here's the locking pin in the frame..... in the locked position .... here's the locking pin in the frame...... in the unlocked position .... here's the locking lug under the barrel...... I held the barrel in a padded vise, and gently removed material equally, (count the number of strokes with the same pressure) from both of the flats on the locking lug, until the force required to lock the new barrel in place was the same as the force required to lock the original barrel in place. Depending on how much initial force is required to lock the barrel in place, you may need to start removing material with a fine needle file at first, then progress to an India stone, then to an Arkansas stone. My barrel was pretty tough to lock before I stoned it, but that metal is not very hard, and was not at all difficult to remove enough metal to allow the same lock-up as the original barrel. I did use a small padded vise to hold the barrel, so that my stones were at a 90 degree angle to the flats I was working on. Disclaimer...... Don't get in a hurry. Check the fit every few strokes. once I got the black finish off the lug surfaces, I darkened the flats with a magic marker to show me any high spots, or angularity. I'm not responsible for anyone else's success or failure regarding this procedure.