'moving the point of impact to the point of aim'
This I understand and agree with but I remain very curious what happens to the pattern? I'm sure its different for each gun and neither the point of aim or impact have anything to do with the pattern. I also am curious if different shells have different points of impact. I don't remember ever coming across anyone who has done any testing.
I can only answer this based on the limited experience I've had, but that says that you are right. In every case, it opened up the pattern to some extent.
What I used it for was to regulate turkey loads in SxS guns. I wanted one barrel to be very tight, one to be more open, and both of them to shoot to the same spot. In some of those cases there was nothing wrong with the gun; it was just regulated for field loads and I was using extremely heavy loads. That made the right barrel shoot right and the left barrel shoot left.
Filing the open barrel to move it all the way to where the tight barrel was hitting seemed to turn a modified choke into an IC, which was ideal for the loads. I also corrected a cross firing gun that was absolutely worthless for anything prior to the filing. That gun has fixed chokes; all the others I've done have tubes.
It is indeed true that you are regulating it to shoot one particular load if you do this, but all SxS guns are regulated for one general load. The difference may not be enough to matter with most loads, but I'm convinced there is always some difference.
Filing a choke is something best done on a gun with tubes. Ruin it and you are out $20 and can easily replace it. I have only done it on cheap guns that were strictly shooters to me. Stan's gunsmith's method sounds like it would be much better on a better grade gun. Brileys can make eccentric chokes that work on the same principle, but my limited experience with them is that they also degrade the pattern and it still may be off. I found filing and testing often to be the best way for me to regulate a turkey gun.
