mechanism of action
http://www.acronymfinder.com/MOA.html

minute of arc, or arcminute or minute of angle (MOA), is a unit of angular measurement, equal to one sixtieth (1/60) of one degree.[1] This conversion is recognised for calculations involving SI Units.[2] Since one degree is defined as one three hundred and sixtieth (1/360) of a circle, 1 minute of arc is 1/21,600 of the same. It is used in those fields which require a unit for the expression of small angles, such as astronomy, navigation and marksmanship. One arcsecond =1/3600 of a degree.

MoA = Minute of Angle

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Note added at 9 mins (2012-03-04 13:57:09 GMT)
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http://en.wikipedia.org/wiki/Sniper_rifle

A military-issue battle rifle or assault rifle is usually capable of between 3-6 minute of angle (MOA) (1-2 mrad) accuracy. A standard-issue military sniper rifle is typically capable of 1-3 MOA (0.3-1 mrad) accuracy, with a police sniper rifle capable of 0.25-1.5 MOA (0.1-0.5 mrad) accuracy. For comparison, a competition target or benchrest rifle may be capable of accuracy up to 0.15-0.3 MOA (0.05-0.1 mrad).

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http://en.wikipedia.org/wiki/Minute_of_angle

The arcminute is commonly found in the firearms industry and literature, particularly concerning the accuracy of rifles, though the industry tends to refer to it as minute of angle

MoA stands for Minute of Angle