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.
About NATO milliradians
One NATO mil is 1/6,400th of a circle, or 0.05625°, so a degree is about 17.78 NATO mils.
A NATO mil is close enough to a true milliradian (6,283 of those make a circle) that gunners treat it as 1 meter of width at 1,000 meters of range. A circle of 6,400 also divides neatly into quarters, eighths and sixteenths.