The degree of choke stamped on a shotgun's barrel means, in all actuality, very little. Take my modified choked waterfowl gun, for instance: it does shoot modified patterns with steel 3s, but tightens to extra-full when I load it with Ts. The only way to know for sure how a particular gun behaves with different loads is to shoot it at paper targets. Test patterning will help you hit more birds and cripple less.
With a minimum of materials and effort, you can learn a great deal about your gun's performance in a single afternoon. You'll need about 40 yards of open space, a safe backstop, and a patterning board. Patterning boards can be elaborate or simple; a whitewashed steel plate set on posts is traditional at shooting schools, but a 4-foot x 4-foot sheet of plywood propped upright works fine. Me, I use a big cardboard box anchored to the ground with a stick. Whatever you choose for a patterning board, it must be able to hold a sheet of paper about 40 by 40 inches.
Rolls of butcher paper or brown wrapping paper make a good source of patterning targets. You'll also need a marking pen, 15 inches of string, thumbtacks, paper clips or a staple gun, a calculator, hearing protectors, and shooting glasses. Finally, you need a calm day to shoot: dealing with big squares of paper in a high wind is incredibly frustrating.
The first step in patterning is to determine where your gun shoots. Set up a target 20 steps away. From a rest or a solid shooting position, fire at least three carefully aimed shots at a single aiming point. The densest concentration of pellets will mark the pattern's center. Ideally, that center will be on or slightly above your point of aim. Many hunters like a gun to shoot above the point of aim, as it provides built in vertical lead on rising birds, while some like a gun that shoots dead on. Whatever your preference, if the gun doesn't shoot where you want it to, visit your gunsmith.
Once you know where the gun shoots, it's time to take a look at how it patterns with various loads. Set up a target, measure off the appropriate distance, take aim at the center of the paper, and let fly. Make a note of the gun, load, choke, and range right on the target itself. Change targets and shoot again. Ideally, you should shoot at least three patterns at the same range with each choke/load combination you want to test. When you're done, roll up all your sheets and take them home. On the floor or kitchen table, find the densest part of each pattern and draw a 30-inch circle around it by tying the string to your pen and using it as a compass.
Count the holes inside the circle, marking each hole as you count it. Divide the number of holes inside the circle by the number of pellets in the load (pellet count information can be found in ammunition catalogs), and you'll get a percentage. As a rule of thumb, you can expect the following percentages from various chokes at 40 yards:
All other factors being equal, shotguns will pattern tighter at higher altitudes and in warm temperatures, due to the lower resistance of the thinner air the pellets encounter.
In addition to figuring percentages, you might find it useful to draw a 20-inch circle around the pattern core, and also to divide the target into quarters. If any section of the target consistently shows fewer pellets than the others, try a different load. If that load also shows a gap in the same section, you may need some choke work done.
While shotguns are customarily patterned at 40 yards, there's no reason to confine your patterning to that range. I shoot pheasants over a pointing dog, so I'm much more interested in what my pheasant loads look like at 20 yards than 40. You needn't figure percentages or even count pellets if you don't want to. Just shoot some patterns at the ranges you expect to take your shots in the field and look for a pattern that's not be so dense in the center as to tear up game, has few gaps a bird could escape through, and has enough pellets filling out the pattern's edges to make hitting easier. For turkey hunting, look for the choke/load combination that crams as much shot as possible into a 10- or 12-inch circle.
Determining your maximum effective range is easy: use the game bird silhouettes available from Hornady and the NRA, or draw your own. Keep shooting targets at increasing ranges. As long as your load will consistently put five pellets of the appropriate size into the bird's body, the target is in range. Turkeys require a minimum of six pellets in the brain and vertebra for clean kills.
A variety of pattern targets on the market will make the job of testing guns and loads easier. Hornady's pattern kit, for instance, provides everything you need: the kit's box folds open to form a disposable pattern board. Two wooden stakes let you fix it in the ground. The kit includes five game bird targets as well as full patterning instructions. The Hornady kit is disposable and is intended to withstand 20 shotgun blasts.
Visible Impact targets are made of polystyrene, and the foam material shows pellet strikes clearly at long range. Two targets are available, a 30-inch circle with a clay target at the center and a smaller turkey target. Both contain full patterning instructions and are recyclable.
Winchester has a couple of pattern sheets also, a turkey kit, and a larger target available with clay target, duck, or turkey silhouettes. The Winchester target also has five different 30-inch circles drawn around it, so if you didn't hold the gun steadily you can still get an accurate pellet count. With any target using a single pre-drawn circle, be especially careful to rest the gun and center the pattern on the target's center, or your pellet counts will be off.
If you use butcher paper, you might want to use the stick-on orange Target Dots available from Peterson Instant Target Company as an aiming point.
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