Convergence can be changed most of the time by selective honing one side of the choke. Think about it this way. As little as a .020 constriction on both sides can make a 50" wide pattern at 40 yards into a 30" pattern. So selective altering one side will change the pattern by several inches. You do get looser pattern when you do this, at least I did.
Figure each .001 should move the pattern about 1/2-3/4" at 40 yards. This is just a guess from two guns I had done. You have to pattern them to make sure that you are getting the change in impact that you expect. First "gun smith" did the wrong side first and we had to do it twice to move the pattern 6" to the right. I guess we went form a full to a improved choke because of the error.
I have read that most double barrels were set to converge at 40 yards. This make sense if you figure that guns were set up to be used at that range. If a skeet gun then you would expect more like 21 yards.
