A mil dot rifle scope is a type of optical instrument used to estimate distance. The scope is a telescopic lens and has a pattern of dots visible through the focus. These dots are used as a basis for calculating range to a target of known size.
The mil dot system was first developed in the 1970s by the U.S. military. The military uses the system for artillery, mortars and sniper rifles. The mil dot system is also found on commercial rifle scopes.
A mil dot rifle scope uses a mathematical measure called a milliradian. The center of each dot, or box or line, depending on the scope manufacturer, is 1 milliradian away from the next dot's center. The length of a milliradian derives from an angular measurement of a circle, which is a complex calculation but fits into a simple formula for estimating distance. There are 6,283 milliradians in a circle, but this measure was rounded up by the American military to 6,400 milliradians and rounded down by the previous Soviet Union to 6,000 milliradians.
The basis of the mil dot system is that the dots or lines on the scope can be used to estimate how far away an object of a known size is from the scope. For example, a sniper can identify a man in the distance as being about 6 feet (1.8 meters) tall, and use his height, along with interpretation of the dots on the scope, to figure out how far away he is. There are a variety of methods hunters or snipers can use to calculate range using a mil dot rifle scope. The scope crosshairs, or the dots, are lined up on the target. The rifle must be held steady for accurate ranging.
The most widely used method is to center the crosshairs of the scope on the target. Centering the crosshairs on the base of the target allows for easy calculations, because the dots can be read up along the vertical line. Then the dots are counted so the shooter knows how many milliradians tall the target is. It is essential that the shooter knows in advance how tall the target is in either imperial or metric measurements. For example, a hunter will generally know approximately how tall a rabbit or deer is.
Once the target has been measured in milliradians, the range can be calculated using a simple formula. This formula is: Target size in yards (or meters) multiplied by 1,000 and divided by milliradians measured = Yards (or meters) to target. For example, a deer that is 1 yard (0.9 meter) tall and measures 1 milliradian on the scope is 1,000 yards (914.4 meters) away. If the same deer measures 2 milliradians in the scope, then he is 500 yards (457 meters) away.
Instead of using the formula, hunters also can refer to a pre-printed sheet of ranges or use a specialized calculator to speed the process. A mil dot rifle scope can also be used for other applications. For example, shooters can accurately predict the position of a moving target when the speed and direction of the target are known, or use the mil dots to adjust their aim after a misplaced shot.