About Gradians
A gradian (also called a gon or grad) is 1/400 of a circle, so a right angle is exactly 100 gradians. It was introduced in France alongside the metric system as a decimal alternative to degrees.
Gradians are mostly found on surveying instruments in continental Europe, which are often marked in gons. One gradian is 0.9 degrees, or π/200 radians.
On a gradian scale a quarter turn is 100 and a half turn is 200, so right angles come out as round numbers. The catch is that 30° and 60° become 33.33 and 66.67 gradians.
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.