To answer that, we first have to know what a mil is.
A mil dot reticle does not refer to the military. The MIL in mil dot is a shortening of the term milliradian. You are familiar with degrees as measurements of angle. You also know from high school geometry that there are 360 degrees in a circle. As the circle grows larger in circumference, the number of degrees does not change, but the distance between each degree along the circle does increase. You may also know that degrees are divided into smaller units called minutes. There are seconds, too, but they are too small for this discussion. A degree is divided into 60 minutes. At 100 yards distance, the angle of one minute is approximately one inch. So if the centers of the two bullets farthest apart in a 100-yard group are about one inch apart, we call that a minute-of-angle group. Get it? At 200 yards, a group measuring two inches center- to-center equals a one minute-of-angle group. At 400 yards, it’s a four-inch group. At 50 yards, it’s a half-inch group.
Now what is a mil?
One mil of angle is approximately 3.6 inches long at 100 yards, and that is close enough to 3.5 minutes of angle to be convenient. In Leapers mil dot scopes, the centers of the dots are one mil apart. If the scope is variable power, this only holds true for the highest power setting.
So, if your bullseye is 3.5 inches in diameter and 100 yards away, it will touch the centers of any two dots next to each other. If it appears only half that size through the scope (from the center of one dot to half the distance to the next center) your target must be about 200 yards away. If the same bullseye spans the distance between the centers of three dots (two with an extra dot between them), your target is about 50 yards away.
Sniper rifles have mil dots on both horizontal and vertical crosshairs so they can measure height as well as width through their scopes. A six-foot tall man is also 72 inches tall. At 100 yards, he would appear to be just over 20 mils tall. At 1000 yards, he would be close to 2 mils tall.
Military binoculars and gunsights are usually equipped with mil reticles. On the standard crosshairs are other short lines that mark mill angles. These are often referred to as rangefinding reticles. To use them that way, you have to know how to apply the correct mathematical formula, plus you have to know the approximate size of your target.
What else can you use the mil dot reticle for? Well, if you are shooting in a crosswind, you can use the dots as additional aim points to compensate for wind drift. If you notice the strike of your pellets in relation to the dots, you can aim off to one side by placing a dot along the horizontal reticle over the target instead of the crosshairs. By choosing the correct dot, you can easily adjust for how much the pellet will drift in the wind and end up with a perfect shot every time. Better still, there is no math involved!