Pattern Analysis For Busy Sportsmen by Ron Jones

W.W. Greener discussed shotgun patterns of 30″ at 40 yards in The Gun And Its Development, first printed in 1881. Then, as today, understanding the relationship between pattern efficiency and gun performance was critically important to the sportsman.

t must have been in the early 1960s when I purchased a Ted Williams Model 20 pump shotgun (Sears, J.C. Higgins) with a “Poly–choke”–style adjustable–collet choke. I became reasonably proficient at skeet targets with the choke set at “Skeet” but was unable to obtain any consistency at longer–range targets with the “Modified” or “Full” settings.

Frustrated, I researched gun references and discovered I could test the patterns of my Model 20 by averaging ten 40–yard shots on paper. All I had to do was cut ten sheets of 36″–square paper, mount them on a vertically supported plywood board, shoot one pattern at each sheet at 40 yards, draw a 30″ circle around the highest concentration of shot in each pattern, calculate the average number of pellet strikes within those circles and, finally, express that number as a percentage of the number of pellets present in the load.

Well, that took all morning, but the result was worth my effort. The Sears collet–type choke was throwing weak, patchy 35% patterns at the Modified and Full settings. From that point on over the past 50 years, I’ve pattern–tested nearly every shotgun that took up residence in my safe. But over time the drudgery and consuming a full Saturday morning just to evaluate a choke forced me to look for a simpler method. What I learned by numbering my patterns in sequence and then critically evaluating what I observed was this:

(1) The first two patterns of a 10–shot series, when fired through a clean/oiled barrel, were nearly always more open than the following patterns. It didn’t take too much research to discover most chokes are more efficient when the internal surfaces are fouled and free of any lubricant.

(2) The pattern percentages from the individual patterns in the 10–shot group would usually span a full choke designation (about 10%–12%), and the first three patterns (after I had discharged two “fouling” shots) almost always produced a composite average within 2–3 percentage points of the value arrived at using all 10 patterns.

I then asked myself: Do I really want to spend my entire Saturday morning shooting and evaluating 10 patterns when I can come within 2–3 percentage points of the 10–shot average by shooting just three patterns? The answer was obvious.


Counting individual pellet holes can be laborious. Limiting your analysis to just three patterns will dramatically reduce the drudgery. Dividing the 30″ circle into an outer ring and inner core will also make it easier to keep track of your count.

Since the individual patterns produced by even the best chokes usually deviate 5% to 6% from the average, why consume all that time trying to determine the exact average for the group? If I just shot one pattern, I would, at most only miss the group average by about 5%. What I really wanted to know is whether my Modified choke (60% optimum performance) is producing patterns in the neighborhood of 55%–65%. Averaging three patterns could do that effectively.

Shot–Range Pattern Analysis

The final refinement of my pattern analysis system was arrived at when I obtained a copy of Oberfell and Thompson’s book The Mysteries of Shotgun Patterns (Oklahoma State University Press, Stillwater 1957). A very convenient table in that book — arrived at by shooting thousands of shotgun patterns at varying distances — gives pattern percentages for all degrees of chokes for all distances (see below).

Oberfell & Thompson’s

Pattern Percentages for All Degrees of Choke for All Distances

Pattern Percentages Distance in Yards

Choke 15 20 25 30 35 40 45 50 55 60
Full 100 100 100 96 82 70 58 48 39 32
Modified 100 100 96 83 71 60 49 39 32 26
IC 100 96 83 71 60 50 41 32 26 21
True Cylinder 98 83 71 59 49 40 33 26 21 17

It is interesting to note these two unique properties of that table: 1) within normal hunting range (20–50 yards), an increase of 5 yards decreases efficiency by approximately 10 percentage points for the same choke; and 2) an increase of 5 yards requires changing to the next higher choke to prevent a loss in pattern efficiency. A rule of thumb based on that data makes it easy for a shooter to make ready comparisons of pattern efficiencies at various practical distances. The rule is: Allow an increase or decrease of 10 percentage points for a corresponding decrease or increase of each 5 yards (see Rule of Thumb chart below).

Oberfell & Thompson’s

Rule of Thumb Estimate Pattern Percentages for All Chokes & All Distances

Pattern Percentages Distance in Yards

Choke 15 20 25 30 35 40 45 50 55 60
Full 100 90 80 70 60 50 40 30
Modified 90 80 70 60 50 40 30 20
IC 100 90 80 70 60 50 40 30 20
True Cylinder 90 80 70 60 50 40 30 20

Because I’m a lousy off–hand rifle shooter, I always found it difficult to keep all of my patterns close to the exact center of a 36″ sheet of pattern paper at 40 yards. Miss the mark by more than 3″, and part of your pattern is off the paper.

Since the tightest choke (Full) is expected to produce 70% patterns at 40 yards, it made sense to me to shoot each choke designation at the distance which is expected to produce 70% patterns. That turns out to be 35 yards for a Modified choke, 30 yards for an Improved Cylinder (IC), and 25 yards for a Skeet/Cylinder choke. If you want to shoot all your patterns at 30 yards (as I frequently do), just evaluate the results expecting 96% patterns for Full, 83% patterns for Modified, 71% patterns for IC and 59% patterns for Skeet/Cylinder chokes. Or use the convenient 30–yard 90/80/70/60 percent groups Oberfell & Thompson provided in their Rule of Thumb table (see above).

Another consideration for keeping patterns on the paper at 40 yards is the little–known phenomenon of shotgun pattern inaccuracy. According to Oberfell and Thompson on page 100 in their book, “Shotgun groups representing the pattern centers will be about one inch in diameter for each 5 yards of distance of the gun muzzle from the target.” That means, even if shot from a benchrest, some shotguns will distribute the center of their patterns as much as 8″ from the aim point at 40 yards. Good luck keeping all your 30″ pattern on a 36″ piece of paper at 40 yards! Even at 30 yards, depending on the vibration characteristics of your gun, you may lose a few pellets off the paper. If your gun is terribly inaccurate, you may want to rethink your gun choice or talk to a gunsmith.

Do you have to pattern all the loads you intend to shoot at skeet, trap, woodcock and mallards? You can, but it will probably not be worth your effort. I use the following rule of thumb: If I’m primarily interested in the patterning characteristics of a gun intended for targets and small upland game, I will pattern No. 7½ shot in a quality load similar in shot volume to the one I will be using on the range or in the field. Handloads are fine, if that is what you normally use. For pheasants in areas where lead is permissible, I pattern shot sizes 5 or 6 (lead) in a quality field load. For non–toxic waterfowl loads, you need to pattern at 40 yards and use the exact load and choke you intend to use in the field.

Field companion

Your field companion may not fully comprehend the intricacies of pellet distribution analysis in July… but she will certainly thank you for assuring pattern density appropriate for the game in December!

What if you don’t always use the same shot size? Contrary to popular belief, the pattern efficiency characteristics of various magnum lead shot sizes is reasonably predictable. If your gun produces 70% patterns at 40 yards using Magnum No. 6 shot, it will probably produce 75% patterns with Magnum No. 2s and 65% patterns with Magnum No. 8s (see Oberfell and Thompson’s table on page 145 in their book). Therefore, if you pattern your gun with the standard target–shot volume for the gauge using the largest pellets you expect to be using, you can reasonably predict how it will perform using a different shot size with that gun and choke. And, within limits, you can reasonably expect a gun with quality chokes to produce similar pattern percentages with a wide variety of ammunition.

In a personal test I ran, I patterned ten different factory/handloads in shot sizes ranging from No. 5 to 7 in the Modified barrel of a circa–1941 Winchester 16–gauge Model 21. At 30 yards, using my 3–shot pattern analysis, the eight different factory loads produced pattern percentages ranging from 80% to 86%, and the two handloads of magnum lead and unbuffered bismuth produced pattern percentages of 81% and 85%, respectively. The average of the 30 patterns (all brands and all shot sizes) was 84%, and no individual load varied from that average by more than 4%. Factory brands included Federal, Fiocchi, Gamebore Traditional (fiber wad), Kent Ultimate, Lyalvale, Remington Dove & Quail, Remington Express and Rio.

That phenomenal consistency indicates: 1) the Winchester chokes of the era produced amazingly consistent patterns, regardless of the ammunition; and 2) the 3–shot pattern analysis did a remarkable job of predicting the pattern percentage for any given load.

What if your gun comes with five choke tubes? Easy. Assuming the choke tubes are of high quality, test the Modified choke with three patterns of No. 6 or 7½ shot. You can reasonably assume if the Modified choke tube produces patterns in the neighborhood of 80% at 30 yards, the other (same brand) tubes will produce patterns consistent with their choke designation, too. Not always true, but, in my humble opinion, it’s not worth the aggravation to determine whether the Light Modified tube is producing patterns right on specification. In order to make that fine distinction, you would have to average at least 10 patterns from each choke. Naturally, if you are primarily a trapshooter, you should pattern the Full choke tube. And if all you shoot is skeet, don’t bother to pattern the gun!

16–Gauge Ammunition Average for 30 Patterns at 30 Yards*

Load Shot
Weight (gr.)
% in
30″ Circle
Ext. Spread
Hi–Lo (%)
handload 1 oz.
437 5 208 85 10
Federal 2½–dram 1 oz. 438 6 231 81 7
Fiocchi 2½–dram 1 oz. 430 6 211 86 1
Gamebore Traditional
(felt wad) 28–gram
426 7 306 84 3
Kent Ultimate 2¾″ 1 oz. 454 6 200 86 2
handload 1–1/8 oz.
492 6 270 81 14
Lyalvale 2½″ 7/8–oz. 374 6 226 80 13
Remington Dove & Quail
2½–dram 1 oz.
457 6 225 85 12
Remington Express
3¼–dram 1–1/8 oz.
468 6 218 84 13
Rio 2¾–dram 422 7 288 86 18
Average 84 9
*Shot through the Modified choke of a circa&ndash1941 Wichester Model 21. Oberfell & Thompson criteria states a Modified choke will place 60% of its pattern in a 30″ circle at 40 yards and 83% of its pattern in a 30″ circle at 30 yards. NOTE: One errant Rio pattern was 95%, explaining the 18% extreme spread.

Steps In Pattern Analysis

Pellet count

Determining the number of pellets in the loads you will be testing will make your test results more meaningful. Wasting one cartridge to arrive at that number is worth the extra time it takes to pop open the crimp, dump the pellets on a paper towel and count the contents.

  1. Determine the number of pellets in each load. I just dump the contents of one load on a piece of paper towel and count by fives. There is no need to count more than one load. You can use the figure listed in published pellet–count tables in a pinch, but you may be introducing a significant error. Factory loads, particularly those of foreign manufacture, can vary by 5% or more from the labeled shot weight, and the actual size of the pellets may deviate from the label on the box by a full size. (I provided a count for some 16–gauge loads above.)
  2. Cut a template exactly 30″ in diameter. Cut out the center to make it easier to position it on the pattern. This is a one–time investment of your time and effort.
  3. Make sure your shooting distance from the patterning board is exact. I like to have the end of the muzzle exactly 30 measured yards from the patterning board.
  4. Shoot two fouling shots downrange to season the barrel.
  5. Mark an aiming point on three separate sheets of pattern paper and shoot three patterns. Write all relevant information (date, distance, gun, barrel, choke, load, etc.) on the upper right–hand corner of the paper and save it to analyze later.
  6. Use the template to circumscribe a 30″ circle around the highest concentration of pellets in each pattern. Your aim point will probably not be the center of the pattern.
  7. Use a felt–tip marker to count the number of pellets in each of the three 30″ circles. Average the pellet hits from your three patterns and divide that value by the number of pellets in the load.
  8. Compare that value to the Oberfell table above.
  9. You’re done!

Counting individual pellet holes is laborious. By limiting your analysis to just three patterns, you will dramatically reduce the drudgery. Understanding the relationship between pattern efficiency and gun performance is critical to good performance in the field. By combining my modified pattern analysis system with the information provided by Oberfell and Thompson in their excellent book, you can save a lot of time and headaches and free up your Saturday afternoons for more enjoyable things — like actual shooting!

Sure, this system is not as accurate as shooting all test loads at 40 yards and analyzing ten shots with each and every load and choke you plan to use, but it will give you a good idea just what your gun and loads are doing, and it is a whole lot better than doing nothing at all — which is what most shooters who hate the drudgery of pattern testing do. And it pays off when you are in the field and can have confidence in the loads you are using because you know you tested them ahead of time. Now, go out there and enjoy blasting birds — clay or feathered — out of the sky!