The mil used here is an artillery and military unit of angle. In this system a full circle is divided into 6,400 mils, which makes one mil 0.05625 degrees.

Mils are handy for aiming because at long range one mil covers about one meter of width for every thousand meters of distance. Gunners and riflescope users can turn an observed width straight into a range estimate with simple multiplication. Note that other trades use "mil" for a thousandth of an inch, which is a length, not an angle.