Milliradian

Milliradian
The PSO-1 reticle in a Dragunov sniper rifle has 10 horizontal lines with 1-mil spacing, which can be used to compensate for wind drift or for range estimation.
Unit information
Unit system	SI derived unit
Unit of	Angle
Symbol	mil or mrad
Named after	The metric prefix mille, Latin for "one thousand"
In units	Dimensionless with an arc length one thousandth of the radius, i.e. 1 mm/m or 1 m/km
Unit conversions
1 mil in ...	... is equal to ...
radians	1/1000 radian
turns	1/2000π turn
gons	≈ 0.063662g
degrees	≈ 0.057296°
arcminutes	≈ 3.4377′

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 targets at different distances. Using optics with mil markings in the reticle one can 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. There are other definitions used for land mapping and artillery which are rounded to more easily be divided into smaller parts. For instance there are compasses with 6400 NATO mils, 6000 Warsaw Pact mils or 6300 Swedish "strecks" 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.