Im always in favor of alterations which can easily be reversed. Id buy an extra choke tube and alter it .003-.005 on one side. Pattern it then take more until you had what you need. You would of course need to mark the tube so it went into your gun the exact same way every time and was not over tightened or your alterations would change you correction in a different area depending on where it ended up when screwed in. Like 9:00 one time and 11:00 or 7:00 which would drive your results batty.
For that matter start switching tubes and checking them all to see where the hit. It might just be the he modified or could just be the one barrel. Rule out simple to things first. Bad tubes, over tightened, out of round. That type thing.
Jon,
Based on my little experience with moving the pattern with muzzle work, I don't think .003-.005 would make a notable difference in this case. My thought is some number like .015-.025" is more likely required for this situation.
