40 yards and a 30 inch circle is the standard. This is toward the end of the useful range of 12 gauge target loads from normal chokes. The skeet choke is tested at 25 yards, toward the end of a useful range.
Pattern testing is only for comparative purposes, so testing at close range is a waste of effort, as most guns will give very high percentages at close range. Already a known quantity.
You need to push toward the ends of efficiency to find maximum performance.
Some will say "I pattern at the distance I break the targets" and I wonder what is the point. Unless it is done at 40yds there is no reference for comparison. Others shoot at a closer distance to simply puff up the numbers.
Johnny, patterning at the distance you break, on average, your targets lets you optimize your patterns for that distance. Shotgun Insight and Pattern Optimizer are invaluable tools in that regard. However, if you do it the old fashioned way, put in the choke tube that gives you 85% PE at target distance. Much less and you run the risk of losing a perfectly pointed bird. Much more and you are losing effective pattern area unless you are a very good shot.
Patterning at 40 yards tells you what the choke is. Useful info, but not as useful as knowing what is happening at the target.
This is an example of how useless counting pellets is. If you shoot your pattern at 40 yards, is that for a 20 yard handicap or 27 yard handicap?
7 yards difference. So if it applies to a 27 yarder, does that mean a 20 yarder should pattern at 33 yards? or if it applies to a 20 yarder, does a 27 yarder pattern his handicap load at 47 yards? 52 yards?
Since most 27 yarders are experienced shooters, do they shoot faster? so maybe 40 yards works? But not all are quick shooters, some are very purposeful. 55 yards?
What if you're a 27 yarder on station #5 looking for a hard right, and you get surprised by a straight away (early morning east rising sun, cloudy, or just not paying attention), and have to swing back hard left? Would that be a 40, 45, or 50 yard target? What if you're a 24 yarder? How far out is that target?
1 or 2 eyed shooter?, 3rd year shooting 13th, or 23rd?
A little history first. The 40 yard 30 inch circle was the standard used to determine the choke. 70% of the pellets in the circle ment that you had a full choke. This system was developed when all shotgun shells were made without plastic wads. All the barrels marked Full choke had .040 constriction. Now we have plastic wads of varying designs, barrels over bored by varying amounts and chokes marked Full usually have .035 constiction. I say that this system is obsolete. If you want to check the effectiveness of your pattern try 32 yards for singles and 16 + what ever your handicap yardage is. If you can have some one stand to the side of the trap range and visually mark where you are breaking the targets then go and measure distance from the mark to the trap.
Now just to be sure that we are on the same page. You mention testing shot pattern. Do you actually mean where the Point of Impact (POI) is relative to the Point of Aim (POA) or actually how effective the pattern is?
Chip, as part of another project I had people stand well of to the side of a trap field and spot where targets were being broken. That, and a little trig told me how far from the muzzle targets were breaking. I found the average ATA trap shooter was breaking 16 yard targets much closer than the 34 yards that is often bandied about. At least at our club, anything over 30 yards was a slooow shooter. Mine were breaking at an average or 25-26 yards. I'm slower now, so I think 30 is about right.