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How To Pattern Test A Shotgun:
 Perhaps one of the most critical parts of shooting an imparticular load of shot out of your your shotgun at a patterning board is the fact you will also know its exact point of impact with your imparticular sight picture. You can have an excellent shot pattern but if it is not where your aiming then its not worth very much from a hunters or shooters perspective. Shot patterns are only part of the overall performance of a shotgun when shot patterning.

 To properly pattern test a shotgun, an exact aiming point is needed for pattern testing a shotgun and there are three very distinctive points of aim for the shotgun. I can not express this enough as this is so critical, you need that pattern exactly where you are aiming your shotgun. The very idea of patterning a shotgun is to make better use of a usable shot pattern and a usable shot pattern is where you are aiming your shotgun for the shot impact zone, on center and on target.

 To correctly aim your shotgun you also have to understand that a shotgun has a sight on it, the same as any firearm, front sight is the bead at the top front of your shotgun barrel. The back sight is the top center of either a flat rib or grooved rear receiver of the shotgun. Correct sighting will show that the bottom of the bead front sight just touching the flat area of the receiver or rib, anymore than this either way and you will shoot high or low. The top of the bead is the center of the shot impact point.

 Various older fixed choke shotguns and some newer ones may have a variation in hitting higher or lower than point of aim, this is common, as the shotgun may have been set up by the manufacturer for a specific purpose at a different yardage.

How Choke Is Determined By Patterning:
 Choke is roughly determined for all shotguns by the amount or percentage of shot it delivers within a 30" circle at 40 yards.
Cylinder bore will deliver 40 percent of its shot load within a 30 inch circle at 40 yards.
Improved cylinder choke will deliver 50 percent of its shot load within a 30 inch circle at 40 yards.
Modified choke will deliver 60 percent of its shot load within a 30 inch circle at 40 yards.
Full choke will deliver 70 percent of its shot load within a 30 inch circle at 40 yards.

Portable Patterning Board Set-Up:
 1. You will have to make a patterning board that is portable as most shooters use public ranges and some ranges will have patterning boards for shotgunners most will not. Making an easel type patterning board is fast, easy and effective.
Building Easel Type Patterning Board, How To:
First, purchase 1 = 4'x8'x1/2" plywood.
Second, purchase 2 sets of hinges, 4 hinges total.
Third, purchase 32 = 1/2" wood screws for hinges.
Fourth, purchase short piece of rope around 3' length.
Construction, cut plywood in half so that you have 2 = 4'x4' pieces, attach hinges to your two halves of plywood so that your easel will fold flat for transportation, while its folded flat, drill a hole through both halves center and bottom of easel, run rope through the holes, tie a knot on one end of the rope, slowly open easel until your satisfied with its stability, tie another knot so that the easel opens the same everytime, you are now done and ready for pattern testing.

Making Patterning Board Targets:
2. You will now have to locate a source for good heavy paper to make patterning targets. Natural kraft wrapping paper of 50 lb. basis paper thickness weight that ships from and sold by works excellent, it is durable and versatile heavyweight paper with a smooth side that marks easily with a felt marking pen. This paper comes in a 48 inch wide roll that is 200 feet long, for around $20.00, pretty cheap if you ask me, thats 50 targets that measure 4 feet square for around $20.00.
 For a more accurate sight picture place a large plus sign centered on your patterning paper for your aiming point, also mark target top as top for analyzing later with correct target orientation.
See Image Below:

How To Pattern Test:
3. Place your patterning board and patterning target set-up at the distance you intend to pattern your shotgun, be sure that your down-range area is clear and safe to shoot. Wear ear and eye protection while shooting.

4. Go to your shooting position, load your shotgun with one shell at a time as you will need to shoot at least 3 different targets for pattern average and you don't want to be down range with live ammo in your shotgun back at your shooting station.

5. Line up your sights correctly and fire one accurate round. Clear your spent shotshell cartridge case, leave your gun action open and go down range and remove your target and put up a new fresh target. Repeat this procedure for at least three rounds of ammo.
  If you are going to try several brands of ammo or different shotshell pellet loads, fire three rounds of each, after each three clean the barrel to remove any plastic fouling that may be in your barrel and choke so you have a good control test of each variation.

6. After you have completed your range session its time now to analyze the results, using a piece of string, a ruler and a pencil draw a 30 circle around the densest part of the pattern on each target. Orient all the targets as they were shot, this should be easy as when you made your targets you marked all of them as to top earlier.

1. For point of impact results, compare the center of the plus sign you had placed on target earlier with the actual center of the pattern you shot. Determine the differences in elevation and windage. Now you know where your shotgun is putting all that shot after it leaves the barrel, very good thing to know.
A. If there is a difference in elevation that is not to surprising, as all shot does drop over distance, this is common though, as the shotgun may have been set up by the manufacturer for a specific yardage.
B. For shot impact zones that show some slight variation in windage, left or right impacts, shot can be moved by strong winds, especially crosswinds. If this was not the case you can have slight variations in windage impacts, if it is very much you may want to consider taking your shotgun to a gunsmith for a professional analysis.

2. For pattern percentage results or choke proofing, count all the pellet holes inside your 30 inch circle. Now you need to know how many shot pellets were in the shotshell you used, try here first.
Simply cut open one of the shells you have left and count the pellets.
Divide the pellet hits in your 30 inch circle by the total pellets in the shotshell that you were using and this gives you your pattern percentage for the choke you used. Do this with all your targets add the aggregate for overall average.
Choke Determination:
 Idealisticly we are looking for shot and choke combinations that produce the following results when patterning.
Extra full choke delivers a 30 inch, 70% pattern at 45 yards.
Full choke delivers a 30 inch, 70% pattern at 40 yards.
Modified choke delivers a 30 inch, 70% pattern at 35 yards.
Improved cylinder choke delivers a 30 inch, 70% pattern at 30 yards.
Cylinder bore delivers a 30 inch, 70% pattern at 25 yards.

For a more detailed shotgun pattern analysis, try here.

Resources And References
Guns Directory:
Shotguns For Sale
Shotgun Choke Tubes For Sale
Shotgun Shells For Sale
Shotguns Guide:
Shot Patterning
Shot Patterning For Turkey
Shot Pattern Analysis
Shot Pattern Improvements
Free Clay Patterning Target
Free Dove Patterning Target
Free Turkey Patterning Target