Choke is very roughly determined by shotgun manufacturers for all standard bore 10 gauge, 12 gauge, 16 gauge and 20 gauge shotguns by their pattern or the amount of shot they deliver within a 30″ diameter circle at 40 yards.
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.
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 every time, 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. The one pictured above will suffice as it is durable and versatile heavyweight paper.
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.
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.
Learn how to make shotgun pattern improvements, here.
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.
Idealistically, 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.
Shot Pattern Analysis:
Choke is very roughly determined by shotgun manufacturers for all standard bore 10 gauge, 12 gauge, 16 gauge and 20 gauge shotguns by their pattern or the amount of shot they deliver within a 30″ diameter circle at 40 yards:
Cylinder bore = 40 percent.
Improved cylinder choke = 50 percent.
Modified choke = 60 percent.
Full choke = 70 percent.
While this tells what choke you have, the really big question of, how efficient is the choke and is there a problem inside of the barrel, remains.
Learn more about shotgun choke selection, here.
Using A Lockhart Pattern Gauge:
Our first 30 inch circular gauge seen below can be made from a piece of Plexiglass that is 30 inches x 30 inches square. This will give you your choke rating and to help identify problems within the barrel and choke, just simply lay the gauge on your already shot pattern targets for analysis.
Determining The Choke:
In a 30 inch diameter circle there are 706.50 square inches of area that needs filled evenly with a small amount of shot, by breaking this area down into 2 even areas we find that inside that 30″ diameter circle is another circle that is 21.21″ in diameter both having equal area at 353.25 square inches each.
This means if you were using a 1 1/8 ounce load of #6 lead shot which has approximately 253 pellets or pieces of shot, to shoot a true 100% full choke pattern it would deliver 175 pellets spaced out at one piece of shot every 4.037 square inches.
1 1/8 ounce load of #6 lead shot = 253 pellets, 70% = 175 pellets, 4.037 square inches, (4.037 sq. in. = 2.009 in. x 2.009 in). Make A Score Card For Lockhart Pattern Gauge:
Just count the shot pellets in each numbered and lettered section of the gauge and enter that number of shot on score sheet.
Column 1 represents the core of the pattern its area is 353.25 square inches.
Column 2 represents the outer ring of our 30 inch diameter circle its area is 353.25 square inches.
Example: Column 1 (100) + Column 2 (75) = total number of shot within the 30 inch diameter circle. 175 total = Full Choke
Assigning A Percentage Value To Each Piece Of Shot:
175 pieces of shot = 100 % of the pattern so each piece of shot is assigned a value of .57 %.
Column 1 = 100 pieces of shot x .57 = 57%.
Column 2 = 75 pieces of shot x .57 = 43%
In this example our full choke has a 57/43 rating.
Identifying Bore And Choke Defects:
Make another score card for Lockhart Pattern Gauge and count the shot pellets in each numbered and lettered section of the gauge and enter that number of shot on score sheet.
If you look at the pattern gauge you will notice that each pie shaped piece is numbered and lettered to reference opposing sections of the shotgun barrel. The reason for this is for each defect in the barrel, it will affect that same area on the pattern and the opposing side of the pattern.
Example: We’ll say that the numbers are fairly even in sector 2, 3 and 4 but in sector 1-A and 1-C the shot count is a lot higher and in sector 1-B and 1-D the shot count is a lot lower. The reason why this happens is because somewhere in the barrel or choke there is a defect that disrupted the shot from one side to the other such as a dent or rust cavity and even new barrels will show where something has been attached to the barrel using heat and causing ripples in the barrel such as ribs and barrel lugs.
Choke Overall Effectiveness:
The below gauge is made to indicate the overall performance of the choke remember we are working with a 30 inch diameter circle or pattern of which the area is 706.50 square inches.
These gauges are made to each shot size, load size and choke configuration, the one below is for our full choke 1 1/8 ounce load of number 6 shot, each square area is 4.037 square inches or 2.009 inches x 2.009 inches.
Once again we are dealing with 175 pieces of shot so our percentage value is .57% for each square area, by simply laying this gauge over the pattern paper you count all the square areas that have no shot in them and multiply by .57%, subtract that number from 100% and this gives you your overall choke effectiveness.
We’ll say that there are 40 empty spaces, multiply 40 x .57 = 22.8 %, subtract 22.8 from 100 = 77.2 % effective.
The Final Analysis:
Our final analysis for this example is, we have a full choke pattern that is 77.2 % effective. Pattern density is 57 % inner half x 43 % outer half of the total square area of a 30 inch diameter pattern.
Pattern density would be little weak for turkey hunting, and for pheasant and upland use, it is to unbalanced with 50% x 50% as our goal for ideal.
This method was developed by Kim Lockhart, the former owner of this website, before he passed away.
If you’re in the market for a home defense shotgun, check out our guide.