Years back, my gunsmith hunting partner was working with his neighbor--a trap shooter--to move the pattern on his gun. Don't know whether it involved filing. However . . . if you file a gun that's threaded for choke tubes, doesn't that mess up the threads and make it impossible to put the tube back in? Or are you talking about filing the tube?

Builder, I'm hoping your problem is one of a bad tube. That makes the solution a whole lot cheaper and easier. Assuming you don't have any recourse from FAIR.