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.
About US WW2 Milliradians
The US wartime mil was an artillery unit, a rounded milliradian, and a right angle came to exactly 1,000 of them (4,000 to a circle). A true milliradian is smaller: 6,283 fit in a circle.
In World War II American artillery crews set elevation and direction in mils, reading the angle between the target and the gun from an aiming circle.
There are 4,000 US WW2 milliradians in a full circle.