12/8/15 7.5/Blue Dot
NOE 227-80 3 groups avg 2.35"
THE EFFECT OF MINOR ERRORS ON GROUP SIZE
Let “error” mean a deviation from perfection.
Some shooters contend that elimination of minor errors; such as variations in bullet weight or condition, powder weight, bullet or cartridge case orientation; increases accuracy.
Assume that a certain minor error causes an increase in group size. Perfect shots produce groups of a certain size, shots with a particular minor error produce larger groups. Perfect shot groups form one distribution, groups of shots with that particular minor error form a different distribution.
Let the perfect shot groups average 1” , and let different minor errors produce groups that average 1.25” , 1.5” , 1.75” , 2” , 2.25” and 2.5” . For example, the bullet weight minor error might produce groups of 1.25” , powder weight minor error might produce groups of 1.5” , etc.
For five shot groups, when one shot with a minor error is included with four perfect shots, group size will be larger than groups shot with five perfect shots. How much larger?
Minor Average
Error 5 shot
Group Group
Size” Size”
1.25 1.028
1.5 1.061
1.75 1.100
2.0 1.144
2.25 1.188
2.5 1.236
We would expect that minor errors would not be included uniformly, with one minor error in each set of five shots.
We would expect minor errors to occur less frequently, in less than 100% of the groups.
If, for example, minor errors with group size of 2” occur 50% of the time, then 50% of the time all shots are perfect and then average 5 shot group size falls to 1.072” . {It’s ((1.144-1)*.5) + 1.}
Minor errors make small changes to average group size.
See the fascinating EXCEL workbook TWO DISTRIBUTION GROUP SIZE.
Attachment: TWO DISTRIBUTION GROUP SIZE.xls below.
