If not for some error, such as wind or barrel harmonicas or shooter error or case weight variation, all the bullets would go through the same hole. The sum of all the errors, assuming a NORMAL distribution of shot location, is the standard deviation. I model group size by entering imaginary average group size and standard deviation.
In analyzing the ratio: largest group / smallest group, I found that my center fire records for five, 5-shot groups averaged 1.979, close to the expected 1.91; and assumed that this was a fluke-my records being so close to the expected.
Then Larry Landercasper posted his records, averaging 1.980 vs. expected 1.91.
Our records, Larry’s and mine, are with one shooter, in a tunnel. My range has close dirt hills each side, and virtually no wind.
The NRA, CBA and IBS records are substantially larger than expected, have wind, and many shooters..
I suggest that the difference is due to wind, (conditions), and number of shooters. Then conditions and shooter ability have a large effect on group size; maybe larger than we supposed..