OK, I somehow seem forced to respond, but do NOT recommend what I am about to say.

When living in Alaska I had a shooting and hunting friend who really loved German Guns. He was a fanatic about patterning and could not keep his hands off of a muzzle to "work" on.

He used a Dremel tool on many of his nice guns. The patterns did change, but I think some of his mistakes would shoot an "L" shaped pattern. The moral is, you can drastically change the pattern and impact center by screwing with the muzzle.

http://www.bakercollectors.com/