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Taken from modern firearms here

The most common way of describing the accuracy of the sniper rifle is to measure average diameter of the circle, that may be drawn arount the group of bullet holes in the target. Usually, the rifle is fired from the rest with groups of the 5 (or 3) rounds, and then every group is measured. Average group diameter is the most common criteria of rifle accuracy.
Today, the thin line between "good" and "poor" accuracy is usually laid in 1MOA group. 1 MOA (Minute Of Angle) is measure of the angle, that formed with the triangle with muzzle as the top and the group as the base. 1 MOA is roughly equivalent to 1 inch group diameter at 100 yards (91 meter), or to 2 inches at 200 yards etc. So, if you read that rifle XXXX shooths 1MOA groups, it means that at 300 yards this rifle could place 5 or so bullets in circle of no more than 3 inches in diameter. Many modern sniper rifles, when loaded with right ammunition, could shoot 0.5MOA, or even 0.3MOA, which mean 1 inch groups at 300 yards, or 2 inch (50 millimeters!) groups at 600 yards (550 meters).

There is a technical measurement of accuracy, but in airsoft the MOA will be much larger, tightbores will decrease that value, as would longer barrels.