A stock that has been shortened can be made long again by attaching new wood to it. If you can, save the old, cut-off piece, as the grain will match better than this.
Remove the pad or buttplate. If you are going to shorten your stock but retain the same thickness pad, you mark the stock for the amount you will shorten it. If you are going to shorten your stock while at the same time adding a thicker pad, you have to mark the stock a distance equal to the amount you want the stock shorter, plus the difference between the two pads. As an example, if you want to shorten a stock by an inch but retain the same ¼-inch-thick pad, mark the stock one inch shorter. If you want the stock to be an inch shorter after you have added a 1-inch pad, mark the stock an 1-¾ inches shorter. (One inch shorter, plus the ¾-inch difference between the old pad and new.) Stand the shotgun on the floor or bench with the top of the receiver touching the wall. If you want to keep the same pitch in your stock, then you will mark the stock parallel to the floor or bench. Place a strip of masking tape along the stock parallel to the floor, along the amount you want to remove. With a ruler, measure the distance you want cut and mark the tape at the heel and toe. While you have the shotgun standing up, measure the distance of the muzzle from the wall. Save the measurement, as it is your pitch check.
Youth model shotguns like the lower 870 commonly have shorter stocks than standard shotguns.
Take the stock off the receiver and with a straightedge draw a line between the two marks. Draw first with pencil, and then with a felt-tip marker. A moment here for pad dimensions. Your pad should follow the contours of the stock. The farther back it goes, the larger the pad must get. To determine the dimensions of a pad can be very easy. If you are shortening a stock by 1 inch to install a 1-inch pad, then the new pad must be slightly larger (butt to heel dimension) than the wood you are removing. If you are shortening a stock more than the pad thickness (taking 2 inches of wood off and placing a 1-inch pad on, for example) then measure the stock at the line where the new pad will be, and use that as your pad dimension.
In any case, you don't want a pad smaller than the stock. However, if you order a pad too large, you may have other troubles. If the pad is solid, you'll be forever (and knee-deep in rubber grindings) trimming the pad down. Pachmayr Decelerator pads are hollow inside. If you start with one that is too large, by the time you grind it down the hollow may be so large that the side walls of pad offer no support. Every time you fire it, the walls quickly collapse and offer little in the way of padding. Properly fitted, the Decelerator is one of the softest pads around. Improperly fitted, any pad can bring tears to your eyes when you shoot. When you order your pad from Brownells, order one just larger than the width and height of your stock.
With the recoil pads flat and the receivers touching the wall, you can see the difference in pitch between these two guns.
Most of the time, when you shorten a stock, the old pad becomes useless. Rarely will you be able to re-install the old pad, unless you are shortening a stock only ¼ inch or so. As the stock gets shorter, the bottom edge (the toe) gets closer to the top (the heel), yet the screw spacing on the old pad does not change. If you shorten the stock an inch or more, the old screw hole ends up too close to the toe. A screw hole too close to the toe is an invitation for the wood to chip if it is set down too hard. Also, when you drill the hole deeper for the screw to be re-inserted, you may drill out through the belly of the stock. You cannot relocate the screw in the pad.
To cut the wood you need one of the following: a fine-tooth saw and a miter box wide enough to accommodate the stock; a radial-arm saw with a carbide-tipped blade; or a table saw with the same kind of blade. You do not want to be using a regular carpenter's saw. The carpenter's saw will probably chip the edges of the stock, making your job more difficult.
With the miter box, adjust the stop so the line falls right under the blade. Check the fit of the stock in the box. You want to he able to clamp the stock in place on each side, and cut right on the line. Clamp the stock in the box, and saw with just enough force to cut the wood, but not so much you splinter the edges of the cut. Once you have cut through the side of the stock top to bottom, turn the stock over and start cutting from the other side. Wood is less likely to splinter if you cut down into it than down out of it. Splintered edges are very difficult to repair, and you are better off spending time avoiding them than spending time fixing them.
The power saws make the job much easier. With a high rpm and slow feed, the carbide tipped blade will cut cleanly enough that you won't have to sand, and they will not chip the wood.
After cutting, reinstall the stock on the receiver and check pitch. If the pitch has changed, you will have to re-cut the stock to get the pitch right.
Remove the stock from the receiver and clamp it in a padded vise. Use a straightedge to draw a pencil line down the centerline of the butt. On the back of the Pachmayr pad container you'll see the hole spacing and pad dimensions. Take the distance from the top of the pad to the top screw hole, subtract .150″ and draw a line across your centerline. Measure down the distance between the screws and mark another crossline. By subtracting .150″ you ensure that the pad will be higher than the heel and give you pad to remove and blend with the stock. Starting with these rough measurements, we will establish the exact centerline of the stock. Measure the width of the stock along each screw location line. Divide the measurement by two, and measure and mark your centered screw location.
Sometimes you can use the old heel screw location to secure your new pad. Rarely can you use the old toe location. For both screws, check the newly measured location to the old screw hole. When you drill your new screw holes, the wood that remains between the holes must be greater than the hole diameter. If there is not enough wood you must plug the old holes before you can drill the new ones. Drill the old hole out with a ¼-inch drill bit. Mix up some Brownells Acra 20, and smear it on the hole and plug. The plug can be a dowel from the hardware store, or a cut-down duck plug from a Mossberg 500. In the gunshop, we used to sell a large number of Mossberg shotguns for defense use. The last thing someone buying an eight-shot shotgun needs or wants is a duck plug. I ended up with a drawer full of them, and used them for plugging stock holes under pads. Press the dowel into place and tap it home with a hammer. Leave the stock clamped vertically until the epoxy has set.
Once set, cut the dowel as flush as you can and sand the stock smooth. If you haven't already, remove the masking tape. Use a compatible stock finish to seal the end of the wood. If you don't seal the stock, the edges of your stock will gradually warp from the moisture that enters from the bare surface.
Drill your new pad holes. To cut the pad for the screws, press a drift punch into the screw hole from the back side, and use a razor blade to slit the pad. Place a drop of synthetic lubricant on the slit and press the screw through. Rub the threads of the screw with bar soap. Wipe a little of the same synthetic lubricant on your screwdriver blade and turn the screws tight into the stock. You've finished the clean part.
Place three layers of masking tape on the stock flush with the pad. Use a felt-tip marker to draw a zig-zag pattern on the top layer of tape. To grind the pad flush you'll need a belt or disk sander. For your own protection you'll need glasses or goggles, a breathing mask, hearing protection and a bright light. The rest of the room would benefit from a Shop-Vac and drop cloths. Spread the cloths over your bench, gun rack, shelves and anything else that would be a hassle to vacuum. If you can, attach the Shop-Vac to the sander. Stand the light over the grinder or sander so you will have a good view of the work.
Pad grinding is noisy, messy, smelly work that you must do perfectly. You do not want to be interrupted while grinding. If you approach the task with a heavy hand you can “kiss” the stock with the