If you wish to retain the shape of the current butt plate(some are cupped) scribe a line arround the stock using the rear of the stock as a guide. Then a bandsaw cut just outside the line to remove the bulk of the excess stock. A belt sander cuts to the line. A large bench mounted disk sander would work also for a flat pad mounting. Rasps and files will work to remove the wood but take a very long time and it is difficult to keep the butt square. If a curved butt is required the pad(and it's base) must be heated with a heat gun to allow the plastic material to conform to the stock wood. NOTE--the pad used should NOT have a metal reinforcement plate, it makes the bending process more difficult. Curved pads should be bent off the stock and to a bit of excess curve(just a bit!). The screws when applied will straighten it out a bit for a perfect fit. Also when installing a curved pad one of the screw holes will need to be plugged in the butt stock and redrilled using the curved pad as a guide. The curving process will shorten the effective distance between the holes in the pad requiring the effort. Be certain to pull the curved pad tightly against the stock when marking the new screw hole. If a flat installation is needed on a curved butt the line can be applied with a straight edge after determining the pitch required. If no change is needed the line is scribed parallel to the current cutt plate(measure in at top and bottom and connect the marks). Otherwise the pitch is measured and the center of the butt is marked(deepest part of the curve). The amount the butt is to be shortened is marked at the center of the butt line which will establish an 'X'. The gun when held at pitch has a level line drawn thru the 'X' previously set and the wood removed. The pitch of the butt is measured by placing the assembled gun against a wall with the butt against the floor and the distance of the barrel from the wall in measured, usually at the barrels 26 inch length. This will usually give us the ability to repeat the measurement on another shotgun. If a synthetic stock is being shortened the mounting pads cast into the stock for the butt pad screws, are cut away. I epoxy in a plug of wood, usually the cut of from another wood stock. This is epoxied into the opening using epoxy. The stock bolt access hole is drilled, if needed, and the screw hole in the plag are filled. New screw holes are drilled using the pad as a guide.
