If one's ever plotted shot holes' radius from calculated group center in a 100-shot group, they'll notice the following's pretty much what happens:
40% of the shots are inside a circle whose radius is 40% of the extreme spread.
30% of the shots are inside an area from the 40% circle out to the next larger 30% of the group extreme spread radius.
20% of the shots are inside the next 20% radius area.
And 10% are in the outside 10% of the group's radius.
The mean radius of all shots from group center is about 70% of the group's 100-shot radius across the extreme spread; the 40% plus the 30% radius.
This was called the 10-20-30-40 percent rule of thumb for shot groups as told to or read by me decades ago. It ain't 100% statistically perfect, but 95% is good enough for most purposes. Do it with a .22 long rifle at 100 yards; cheap and easy to see what happens.
One other thing... If you shoot several groups with a given load and they're not within 10% of the same size, you don't have enough shots per group to be statistically valid.
|