Patterning your shotgun for point of impact.
There are several reasons why you should pattern a shotgun. Patterning will determine where the manufacturer has designed and engineered the gun to shoot. Patterning will determine the density of the pattern. And lastly, patterning will determine where you will shoot the gun.
The first step I’ll cover will be the guns point of impact. The point of impact is where and how the shotgun delivers it’s payload to the patterning paper.
Now you may wonder "at what distance will I be shooting at". For many years, the standard has been 40 yards. This is generally the average maximum killing distance. This standard is still being used, but with more shooters participating in clay target shooting, sometimes shorter distance are used. Many skeet shooters will use 25 yards for their game. Using these two distances, we will be able to compare guns of the same makes & models, and also guns of different makes and models you could call it, "comparing apples to apples."
Let’s get started. I’ll use the 40yard distance, assuming you will be using this gun for trap shooting or longer range hunting for pheasants or waterfowl. What you’ll need is a patterning board that will hold a 4 foot x 4 foot piece of heavy craft paper. Be also sure that the area behind this patterning board will have a safe area for the shot to fall. A shooting bench or some kind of steady platform will be needed. Remember we are trying to determine the guns point of impact (POI), not how the gun fits you and where you would shoot it. Another piece of equipment that I use is a piece of plexi-glass, 30" in diameter, with a 3/8" hole drilled in the center. It’s been said that pellets within this 30" circle will still have enough energy to kill or break a target.
Patterning a shotgun is not "rocket science", nor is it without its variables. In any given box of ammo, you will have shells that will pattern with small inconsistencies. What we will be looking for is an average that we can be satisfied with.
Now with the patterning board up, the paper attached to it, a 3" dot drawn in the center of this paper and the shooting bench set at 40 yards, let’s take a look at the shotgun being used. Most target guns have two beads; a front bead and a mid rib (rear) bead. Having two beads is big plus during the patterning process. This will allow the shooter to be able to aim their shotgun, just as you would a rifle. Because most shooters prefer to see the bead alignment looking like a figure 8, this will be the site picture we will use. If there is not a mid rib bead, then the front bead will need to be appearing as if it’s laying flat on the rib. You should not see any rib between you eye and the bead.
Loading the shotgun and placing it on the bench, grab hold of the gun and place your head on the stock. While taking aim at the patterning board, it’s very important that you maintain the figure 8, or the bead is laying flat on the rib. You may have to readjust your head placement to get the picture we want. I know you will not be shooting your gun in an uncomfortable condition, but then again, we are determining where the gun shoots. If in anyway you cannot get the sight picture we want, a problem with your normal shooting can be exaggerated.
While aiming at the target, place the front bead at six o’clock of the 3" dot you have drawn on the paper. Holding the bead at this position is a very consistent hold. Allowing the bead to climb up onto the dot creates a variable of how much of the dot are you covering.
Taking one shot at the target will show you the pattern and the POI, but not very well. I suggest taking three shots to make the shot pattern denser. I also like to shoot at five pattern sheets to eliminate any shooter variables and to determine an average.
Laying the pattern sheets out that you have fired at on the floor, take the 30" Plexiglas and lay it over the pattern. By being able to see through the Plexiglas, you can see the pattern. Move the circle around on the pattern until you can get an even distribution of pellet holes around the edge. Place a magic marker dot through the hole of the circle. This dot will indicate the center of the pattern. Do this with all five patterns. Hopefully, all five pattern centers will be close to one inch of each other.
Taking a measurement from the center of the aiming dot to the magic marker dot, this will tell us how high the gun is shooting, and how far to the right or left, it is shooting. There are two ways of looking at these dimensions. You can say that this particular gun shoots "x" inches high and to the side, or as some do, they will look at the pattern as a percentage. To determine a percentage, it will only work using the 40 yard distance and the 30" circle.
If the pattern center is on the aiming point, this is called a 50/50 pattern. 50% of the pattern is above the aiming point, while the other 50% is below. Every 3" that the pattern center is moved upped; the percentage will change 10%. For example, a POI center 3" high will be called a 60/40 pattern. A POI center 15" high would be called 100%. You cannot look at the edge of the pattern to determine the POI, only its center.
What will these patterns tell you? The 50/50 pattern or even the 60/40 (3" high) pattern, will work very well for a hunting gun, sporting clays or skeet. Most of this shooting consist of many more passing shots. Passing shots are generally at more horizontal flying targets or game. Any pattern from 60/40(3" high) or higher, is much more suited for trapshooting. Trapshooting involves a rising target. So to have a gun that shoots higher, gives you some "built-in" vertical lead. This "built-in lead" or higher shooting gun, will allow the shooter to maintain their vision on the target while shooting. If shooting a flatter patterning gun at a trap target, means you’ll have to push the bead up onto target, essentially having to cover the target with the front bead, and losing sight of it. You will be likely shooting over the target, because you may cover it too much.
For the purposes of patterning a shotgun at a shorter yardage, say the 25 yard distance, most shooters are using a much more open choke. An open choke is use primarily by skeet shooters and upland bird hunters, for grouse, quail, and over pointing dogs. The shooting distances seldom go beyond 30 yards, and many of are less than 20 yards. A few shooters will also be shooting smaller gauges. So, 25 yards is a very good average yardage to pattern at. Still using the same 30" Plexiglas circle, the patterning procedure used for 40yard patterning, is going to be used. The board, paper with dot, shooting bench, and sight picture are all the same. The only difference is the distance. Patterning at this shorter distance, most shooters will be using the choke that they’ll be using for target shooting or hunting. This is usually anything from cylinder bore to improved cylinder. A modified choke can be used, but you may find that it will tend to tear up the paper more. Patterns that are relatively flat or a few inches high will serve the above shooting situations the best.
If most of the patterning is done with smaller gauged guns, I suggest going with a smaller diameter Plexiglas circle. A 26" diameter will help; for the pellets outside this 26" diameter, will not have the energy that a 12 gauge gun would have.
Ok, now we’ve determined where this particular gun shoots. How does this help the shooter? First off, we know that the gun shoots straight, neither left nor right, and how high it shoots. Secondly, with the help of a competent gun fitter, he can help with the proper fit of the gun to the shooter. He will take into account, your shooting posture, gun mount, proper placement of your head, and being sure your eye is aligned properly down the center of the rib. I won’t get into the details of fitting. This is best left to the fitter. He can explain the importance of each of the above. He will also take into consideration, the primary use of this particular gun.
Can a shooter deviate from the above patterning? Only once we’ve determine where the gun is shooting. Some shooters prefer shooting with some space between the beads. This is fine, but my suggestion is not to exceed one front bead diameter. Many trapshooters prefer a gun that shoots much higher than what the manufacturer has to offer. So for them to achieve a higher POI, they will either but more space between the beads, meaning they will have a higher comb on the buttstock, or they will have the rib changed or an additional rib added to the gun, with a different taper to the barrel bore’s centerline.
Patterning at different distances, are only useful for the specific range in which most of the shooting is done. This is generally tied to the target shooters, and would fall under custom work. If a shooter knows that most of their targets are broken at a specific range, the gun and/or chokes can be changed accordingly. For the majority of recreational shooters, the patterning procedures from above will be very effective.