Optimum Choke Percentage


Dear Technoid,

You offered the opinion recently that “77% at the desired range is an optimal pattern”, but declined to discuss why this was true. It seems a little counterintuitive to me; I’m more used to something like “maintain an average of 1 pellet per 2 square inches in the effective edges of the pattern at the desired range when trying to break edge-on clay targets”.

For example, suppose I am shooting skeet. At one extreme, if I shoot 1-1/4oz of #9, I suspect that 77% at 22 yards will be such a dense center that one would be better off with a wider spread. On the other hand, if I were to shoot 1/2 oz of #7-1/2, I’d probably need a 90% choke to have any reasonable chance of hitting birds, even when centered. Neither of these is a normal choice of load – maybe the 77% rule is for more reasonable situations?

Thanks for your thoughts (when you have the time),

Pete

Dear Pete,

Yes, it is counter intuitive to think that one percentage number works for all loads at all distances, on all targets and in every circumstance. It doesn’t perfectly for everything under the sun and it’s my fault for not making all of my parameters clearer.

I derived the 77% number from Ed Lowry and Keith Garner’s excellent 1996 ballistics program “Shotshell Ballistics for Windows”. Here’s how it was arrived at. I use the following criteria: the target is an edge-on clay target with 6 sq/in of area showing. I used Warren Johnson’s excellent “Choke Chooser” standards of a 95% chance of a one pellet strike (mathematically equal to an 80% chance to a two pellet strike) as my minimally acceptable fringe density. I have found that Warren’s number is a pretty good “real world” figure. I also used what I felt were “standard” target loads of 1-1/8 oz of #9, #8 and #7-1/2 shot sizes.

By running the Lowry program, if you input the number of pellets in your load and the percentage of that load it puts into a 30″ circle at the distance at which you are shooting (not necessarily 40 yards), you can calculate the probable chance of the range of pellet strikes on the target at any distance from the center of the 30″ circle on out to the edge. Lowry’s program does this because he has programmed pellet distribution to follow the Gaussian or bell curve, a theory first proposed by Journee in the ’20s and reinforced by most modern ballisticians.

When you plug in the numbers for a 1-1/8 oz load of #8s containing 461 pellets and move the center of the target away from the center of the circle until you get a 5% miss probability (thus 95% chance of one pellet hit and 80% chance of a two pellet hit- our fringe criteria), you get the following distances at the following percentage chokes:

40% of pellets in 30″ circle = zero chance of only 5% misses
50% of pellets in 30″ circle = zero chance of only 5% misses
60% of pellets in 30″ circle = 5% misses at 6″ from circle center (i.e. maximum acceptable diameter is 12″)
70% of pellets in 30″ circle = 5% misses at 9″ from circle center- 18″ effective pattern
77% ……….. at 10″ from center- 20″ effective pattern
80% ……….. at 10″ from center – 20″ effective pattern
90% ……….. at 10″ from center – 20″ effective pattern
99% …………at about 9.5% – 19″ effective pattern

Right around 77% is the first place where the maximum effective pattern diameter is achieved. It is interesting that patterns from about 75% to 95% are equally effective in this model with this particular pellet count. The most open pattern is selected because it gives the largest “almost acceptable” fringe beyond the acceptable fringe.

The percentages are roughly the same using a 658 pellet count typical of a 1-1/8 oz load of #9s. Effective pattern spread peaks right round 77%, with 70% and 80% each giving slightly smaller patterns. In this case, that 77% is about dead on.

With the 393 pellet count typical of 1-1/8 oz of #7-1/2s, the model fails slightly with the optimum pattern percentage being 90% (producing a 19″ pattern). 77% produced only a 17″ pattern. With this pellet count, 80% to 99% patterns give you 18″ or better.

So, with the three sizes of shot (pellet count is what matters here) commonly used in clay targets, and shooting at an edge-on clay target, that 77% number seems perfect for #9s, the beginning of the good part for #8s and a bit too loose for #7-1/2s, given my criteria of a 95% chance of a one pellet strike.

Obviusly, these numbers would change drastically if you changed the edge-on 6″ clay target area to a 15″ face on target. You need very much less choke in these situations, but I wanted my number to reflect the most conservative situation.

Then you also have to factor in pellet energy. A two pellet strike from a 1200 mv #9 at 20 yards may do the bird in, but probably won’t at 40 yards where it a two pellet strike from #7-1/2s would be about the energy equivalent. Energies from each of the three pellet sizes equate at about 20, 32 and 40 yards. Obviously, you have a better chance of hitting a 40 yard crosse with a load of 658 #9s, but the energy is such that you will be less likely to break it and so you can’t apply the 95% chance of a one pellet strike as an acceptable parameter. I have seen 60 yard crossers broken with #9s, but that certainly doesn’t mean that #9s are the best pellet for that particular shot.

Anyway, that’s why I picked 77%. I think it’s a pretty good number. I also think that most people under choke for the clay target games. Now how’s that for sticking my neck out?

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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