Looking first at the group for the 1x 7 twist rate, the average BC values for this group of 10 rounds is 0.297 when rounded to three significant figures, sufficient for trajectory computations. The standard deviation (SD) of the measurements, 0.0022, is less than 1.0 % of the average BC value for the group, and the extreme spread (ES), 0.0079, is less than 4.0% of the average BC value. These figures illustrate the criteria that we use (SD no more than about 1.0% of average value, and ES no more than about 5.0% of average value) to determine whether the measured data are “good.” If either of these criteria is seriously exceeded, we look for a reason or repeat the measurements.
The 12 round group for the 1x8 twist rate also has an average BC value of 0.297. The standard deviation for the group is 0.0039, which is about 1.3% of the average BC value, and the ES, 0.0129, is about 4.3% of the average BC value. This group obviously is not quite as “tight” as the previous group, but we would not call this “bad” because the SD does not seriously violate our standard deviation criterion.
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