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MOA stands for "Minute Of Angle," which is a unit of angular measurement. When the accuracy of a gun is given in minutes of angle, it is possible for a person to know approximately what size groups the gun shoots within its effective range.

There are 360 degrees in a circle, and 60 minutes of arc in each degree, totaling 21,600 minutes of angle in a complete circle. Imagine the shooter is at the center of a circle, and that the target is on the edge of that circle. The distance, or range, from the shooter to the target is also the radius [R] of the circle. For any given range or radius [R], one minute is equal to (2) x (Π) x (R) / (21,600).

If the range is 100 yards, that distance is equal to 3,600 inches. When we plug 3,600 in for R in our formula, the result is (2) x (3.1416) x (3,600) / (21,600) = 1.0472 inches. Therefore, at 100 yards, one MOA is equal to 1.0472 inches. The formula works for any range. If the range is 500 yards, one MOA equals 5.2360 inches.

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