  :A radian is a length only if measured along a circle.  If you want the length of one milliradian at 1000 meters, the only way it is meaningful mathematically is to measure off 1/1000th of the range along the circle centered at the shooter.  That gives a height of 1000 sin .001, which is indistinguishable from 1 meter (it's off by a fraction of a micrometer).  