I feel like a voice crying in the wilderness....one more time: Variation does not stack linearly. This is not intuitively obvious. In fact, it is the opposite of intuitively obvious. Yet it has very important consequences for firearm accuracy.
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He ignores the concept of error stacking. If my gun shoots 2 MOA, and I add 2 MOA of wiggle, all of a sudden I have 4 MOA groups, which would be 12" at 300 yds. Add another MOA of wind correction error, and things are that much worse.
This is exactly how practically everyone interprets the situation. However, reality is actually more kind to the shooter.
If your firearm has 2 MOA of wiggle, and you have 2 MOA of wiggle, and there is no wind your wiggles will stack up to 2.8 MOA rather than 4 MOA. If the wind adds another 1 MOA of wiggle, your groups are 3 MOA.
More realistically, if you have 4 MOA of shooter wiggle, and your gun has 2 MOA of wiggle, your groups will be 4.5 MOA. The difference between a 2 MOA gun and a 0 MOA (perfect) gun in this case is a trifle under .5 MOA.
If one of the sources of variation is larger than the rest, it alone almost completely determines the total variation. That is why you have to find the single largest source, and kill it, rather than than fiddle with the lesser sources of variation which will make almost no contribution to total variation.
John is correct in his statements about 3 shot groups. A single 3 shot group has very little predictive power as to what the next group of shots will look like.
I think part of the problem is psychological. Somebody shoots dozen groups, figures that the best of them truly represents the gun, and that the poorer groups were the result of some kind of shooter error. If the shooter does everything perfectly every time, there will still be substantial variation in group size. A single group of (for example) 3 or 5 shots is a poor indicator of future performance. If you shoot enough groups, even the worst sewer pipe barrel will occasionally produce a .5 MOA group.
