Shotgun-Insight 'Perfect' Pattern Generator (Why use it?)
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(c) Written by A C Jones http://www.shotgun-insight.com
This little programme is useful beyond its first impression!
By repeatedly generating patterns with the same pellet count and spread it illustrates how variable patterns can be. It illustrates why it is risky to look at just one or two patterns on a pattern plate and draw a conclusion "by eye".
By altering the spread and firing a few simulated shots one can see at what spread gaps start appearing in the pattern. The spread at which gaps appear can be compared with a measured spread and an assessment made of whether more or less choke would be a benefit.
By altering the spread by 15% or so, one can see that it is very hard to guess the relative spreads "by eye". 15% is approximately a whole choke step!
Altering the pellet count allows one to see why tighter chokes are needed with lower pellet counts. Put in a pellet count of ~280 pellets and you'll see why the .410 makes skeet a challenge! You either have a wide pattern with gaps in the middle or (by 12g standards) a very tight spread.
Put in a pellet count of ~660 pellets and you'll see why a 1-1/8oz load of No.9s is such a benefit for NSSA skeet! It's hard to believe one could ever miss!
To get a feel for how the 75% spread diameter relates to real spreads: My very open skeet gun gives a spread of ~27" at 21yds. My sporter with a "tight 1/2" choke gives a range of spreads of 17"-21" at 25yds depending on the shell. A good full choke trap gun can deliver 26-27" 75% spread diameters at 40yds.
Why use this 75% spread diameter rather then the historically used "pattern efficiency"? Firstly, pattern efficiency is not much use once the figure goes above 90% and once a pattern is at 100% pattern efficiency the measure ceases to be any use at all. If one looks at standard tables, at 20yds the pattern efficiency of 1/4, 1/2, 3/4, and Full is given as 100% even though they give different spreads. The 75% spread diameter reflects the true diameter of the spread and works at all pattern sizes. It also allows patterns to be re-scaled to see the effect of distance on the pattern. A pattern taken at 30yds might yield a 25" 75% spread diameter. Rescaling the pattern by 10% to a 27.5" 75% spread diameter would show an estimate of the pattern at 33yds. With the Shotgun-Insight software this feature can be used to estimate if the gun is over or under choked for a given distance and pattern plate target.