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M=60[ Tan -1 (group size in inches/range in inches)]

where M= number of minutes of angle subtended or 1 min = 1/60 degrees = 1.047 inches at 100 yards

Minutes of angle subtended by a target group at 100 yds.

M = Target group size (inches)/ 1.047

Determining Standard Deviation (target precision or velocity)

s.d. = square root of sigma(dn)2(squared)/N-1

where s.d. = standard deviation(precision = inches, velocity = feet per second)

dn = vertical distance from reference line in inches(precision) or velocity deviation of individual round from average velocity in feet per second (velocity)

N = number of rounds in group

These are a few formulas I use to determine MOA, velocity, and precision of individual handloads. These take a good bit of human error out of measuring groups with a ruler and/or micrometer. There is a complete chapter dedicated to using these formulas in Precision Handloading by John Withers. :-D