How plastic shims can fine-tune your gun—and make you a better shot

The 98 cents’ worth of plastic shims that came with your new $1,800 semiauto might be the most important part of the whole shotgun. Stock shims let you fine-tune cast and drop to get a perfect fit—and make your new gun almost a part of you. This is a big deal. Before shims, altering a wood stock involved a gunsmith and real money, and changing the dimensions on synthetic stocks was pretty much impossible.

Now, many shotguns come with shims, including such affordable standards as the SuperNova pump and the Mossberg 930. No longer is the off-the-rack shotgun a one-size-fits-all proposition. That little bag of plastic shims can make you a much better shot, if you know how to use what’s inside.

Mount Up

The first thing to do with your shim kit is…nothing. Put it someplace where you will not lose it, and start with an honest appraisal of your gun mount. If you practice it on a regular basis and are already dialed in, you can skip this step. Otherwise—and here I am talking to the vast majority of you—listen up.

Your head is the rear sight of the shotgun, but think of it as a scope for a moment: There’s no point in sighting in a gun if the mounts and rings are loose and the scope wiggles. Likewise, if your head doesn’t meet the stock consistently, your point of impact will wander and gun fit becomes a moving target. You need to crank down the screws. With a real scope, you use a Torx wrench. With a shotgun, you practice your gun mount until the “sight” (your head) comes to the same place on the stock every time.

I’ve written ad nauseam about how to mount a shotgun correctly, but it really matters, so let’s review: Practice at home by first checking and then double-checking that your gun is unloaded. Pick a spot on an opposite wall, fix your eyes on it, and bring the gun up so that the muzzle points to the spot without your looking at the bead. Do this by pushing the muzzle toward the target while raising the stock smoothly to your face first, not your shoulder. Don’t crush your head into the stock, because you won’t do that in the field. If you practice this drill with a Mini Maglite AA in the muzzle, the beam will tell you if the gun is pointing where it’s supposed to. You can also check your work in a mirror. When you mount the gun on your reflection, you should see your eye centered over the rib. Do that for 10 to 15 minutes a night for a couple of weeks.

Dial In

Once you are mounting your gun consistently, take it to the range and shoot “groups” with a tight choke while standing 16 yards from the target. Use paper, a steel pattern plate (no steel ammo with steel plates, though) if you have access to one, or a hanging bedsheet with a mark painted on it. Mount the gun, neither rushing nor aiming, and shoot at the mark. Don’t correct if you’re off target. You’re trying to shoot a good group, not hit the bull’s-eye.

If you hit the same place every time, you’re ready to consider shims. (If you don’t, keep on practicing your gun mount.) Look at the center of your group. If it’s on or less than 2 inches off the mark, you’re probably good (depending on where you want your point of aim to be; see below). Otherwise, for every 2 inches off, you need a 1⁄8-inch adjustment to the stock in the appropriate direction.

The shims go where the buttstock meets the receiver, so you’ll need a Phillips screwdriver to remove the pad, and a long flat-head screwdriver or extended socket wrench to take off the stock. (Some kits also include a plate that goes over the stock bolt after you put the stock back on.)

If you want the gun to shoot higher, use a shim that gives you less drop; in the case of Italian shims, it’s a lower number of millimeters. With U.S. shims, it’s usually something like +1/8. (Check the manual.) To lower your point of impact, which is the most common adjustment, use one with a greater number of millimeters or –1/8. Move the pattern right with cast off, left with cast on. Italian shims are marked D for destra (“right” for cast off) and S for sinistra (“left” for cast on, or “evil,” which is completely unfair to us left-handers). Repeat the pattern process and change out shims as necessary until you’re satisfied. When that’s done, I like to go to station 7 on the skeet field and shoot low-house outgoers with a low-gun start to be sure the gun hits where I’m looking.

Where you set your point of impact is up to you. Most hunters and many sporting-clays shooters prefer a gun that centers the pattern on the point of aim, giving a 50/50 pattern that prints half above and half below the aiming mark, and a sight picture that’s flat along the rib. Some upland hunters and target shooters prefer to “float” the bird over the barrel, and therefore like a gun that shoots a little high—about 55/45—and lets them see a little bit of rib. Trapshooters like guns that shoot 60/40 or even higher. However you prefer to shim your stock, you’ll have made it a perfect fit for you, and that’s time and 98 cents very well spent.

GEAR TIP: Beware the Floating Gun Case

Floating gun cases have become standard equipment among waterfowlers and many other hunters. They’re convenient and offer peace of mind should the boat swamp. But they have one huge drawback: They don’t breathe. If you put a damp waterfowl gun in a floating case after your hunt, it might be orange with rust by the time you get home a couple of hours later. It has happened to me. Take the time to wipe your gun down with an oiled cloth before you put it back in the case. By the way, the best method for drying the inside of a floating case is to stick it on a boot dryer.

Written by Phil Bourjaily for Field & Stream and legally licensed through the Matcha publisher network. Please direct all licensing questions to legal@getmatcha.com.