To file the muzzle of a shotgun to move its POI, file the Opposite side from where you want it to move to. ie. To raise the pattern, you want the bottom of the shot column to released from the barrel before the top, so that gas pushes on the bottom edge of the column and raises the column as it leaves the bore, making the pattern hit high. It doesn't matter how thick a wad you have behind the shot in the barrel. The wad(s) is being pushed by the gases, and when the gas can escape from one side of the muzzle before the rest of the muzzle, it will do so, pushing the wads, and column of shot in front of the wad in the opposite direction.
Before doing this, try increasing the powder charge of your load. Try FFFg instead of FFG powder. The faster powder might change the POI on the load. Or slow the charge down. Sometimes a slower load will strike higher as the barrel time is longer, and the muzzle has risen more than when shooting a hotter load.
Change wads. Change lubes. Try lubing the barrel after you seat the wads, and charge. Change every combination of components, before you decide to file on the muzzle. It only makes sense. :hmm:
I only suggest trying everything because many shooters report widely different points of impact when they change one thing or another.
One last thing you can do is to raise the comb of your stock, by gluing pieces of soft leather to the comb to build it up. The raising of the comb raises your eye, which is like raising the rear sight on a rifle. I would tape corregated cardboard on the comb first, and shoot a few patterns to see how much that raises the pattern. Duct tape and cardboard will quickly tell you how much the comb needs to be raised to raise the pattern to your POA. Then, use a table saw to saw a flat on the top of the stock, and take new wood to make a raised portion to the existing stock, to match the new comb height. If you look at some of the expensive Trap Shotguns, they have adjustable comb stocks on them already. You may even decide its worth buying and installing such a device in your ML shotgun. Briley's in Seattle does that kind of work, among others. I have shot with many 27 yard handicap trap shooters who have added wood to the comb of their stocks, to make the gun fit them, and shoot the pattern to their desired point of aim. These are modern shotgun shooters. I can't see any reason not to do something similar to the stock on a ML shotgun. With today's super glues, and epoxies, you can glue a new piece of wood on the stock, and file a new comb. Stain and finish it to match the existing wood and you are in business.
Oh, it doesn't take much filing on the barrel to change the POI, so do a few strokes, after marking the barrel to indicate exactly where you want to take metal off, then shoot the gun to see what change in POI has taken place. Then file some more. Trial and error are the only way to do this particular technique, and you want to keep your errors to a minimum. Best wishes. :thumbsup: