About Milliradians
The milliradian (mrad, or mil) is one thousandth of a radian, about 0.0573°. At 1 kilometer it spans 1 meter, which is why it is the angle unit of rifle scopes and artillery sights.
A milliradian measures an angle, but it converts easily to a size: 1 mrad spans 1 meter at 1,000 meters, 2 meters at 2,000 meters and so on. So a target that measures 2 mrad in the scope and is 1.8 meters tall is 900 meters away.
A circle holds 2000π, or about 6,283, true milliradians. That is an awkward number, so most military "mils" are rounded to 6,000 or 6,400 per circle.
Note that there are several variants of Milliradians, namely NATO Mils, USSR Mils, US WW2 Mils and UK Mils. Information on these variants are available from the links on this page.
About USSR milliradians
The USSR mil was the angle unit of Soviet artillery. It is a rounded version of the milliradian, within a few percent of one thousandth of a radian.
Soviet artillery used the mil in its sights and for estimating range: a target of known width that covers a measured number of mils gives the distance.
Like the other mils it is a military convention rather than an SI unit – each army chose a convenient number of divisions for the circle, so check which mil a figure refers to.
There are 6,000 USSR milliradians to a full circle.