mrad

A milliradian, often called a mil or mrad, is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Mils are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel (up, down, left or right). Mils are also used for comparing shot groupings, or to compare the difficulty of hitting different sized shooting targets at different distances. When using a scope with both mil adjustment and a reticle with mil markings (called a mil/mil scope), the shooter can use the reticle as a "ruler" to count the number of mils a shot was off target which directly translates to the sight adjustment needed to hit the target with a follow up shot. Optics with mil markings in the reticle can also be used to make a range estimation of a known size target, or vice versa to determine a target size if the distance is known, a practice called "milling".
Milliradians are generally used for very small angles, which allows for precise mathematical simplifications to more easily calculate back and forth between the angular separation observed in an optic, linear subtension on target and range. In such applications it is useful to use a unit for target size that is a thousandth of the unit for range, for instance by using the metric units millimeters for target size and meters for range. This coincides with the definition of the milliradian where the arc length is defined as 1/1000 of the radius. A common adjustment value in firearm sights is 1 cm at 100 meters which equals 10 mm/100 m = 1/10 mil.
The true definition of a milliradian is based on a unit circle with a radius of one and an arc divided into 1000 mils per radian, hence 2000π or approximately 6283.185 milliradians in one turn, and rifle scope adjustments and reticles are calibrated to this definition. There are also other definitions used for land mapping and artillery which are rounded to more easily be divided into smaller parts for use with compasses. For instance there are artillery sights and compasses with 6400 NATO mils, 6000 Warsaw Pact mils or 6300 Swedish "streck" per turn instead of 360° or 2000π, achieving higher resolution than a 360° compass while also being easier to divide into parts than if true milliradians were used.

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