here's the formula I use in MS Excel to determine MOA of a given group:
=60*2*(ATAN((A4/2)/(B4*36))*180/PI())
In the case of this exact formula, the group size goes in A4 and the distance in yards goes in B4. Just change the A4 and B4 to whatever cells you want. Copy it & paste it in and it'll start working for you right away. You may have to change the cell references if you have the "R1C1 reference style" set in Excel.
Enjoy.
For the technically minded, you can use the trig functions for a right triangle if you take the isoceles triangle formed by the bore and the group size and split it in half longwise to get two right triangles. Do the trig on one to get 1/2 the MOA, and double it. The tangent of the 1/2 grouping angle is opposite (1/2 the group size) over the adjacent (distance in yards). Use arctangent to get the angle, then do the stuff excel requires to convert from radians to minutes of angle, and bam!, there you are.
Jon
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