MOA and Mils
Remember doubting the usefulness of anything you learned in math class? That comes back to haunt you today. Any rifle is unlikely to shoot center of the target straight out of the box. It needs sighted in. Whether you use iron sights or optics, you need to make adjustments to them for your shots to hit your desired target at a specified distance.
The zero of your rifle is the sight setting where your shots hit center as long as the muzzle does not move, at a specific distance. (You will have a different zero for different distances.) Zeroes are not stable. They change with environmental conditions, ammunition, etc., though the changes are generally minute. How do these changes work? There are two primary systems of measurements – milliradians (mils) and MOA (minute of angle). Both are angular units of measurements and increase proportionally with distance.
Bullets do not fly in a completely straight path, but rather in an arc. There is an angle formed between the bullet impact and the muzzle of the gun. In order to move the bullet impact and maintain the same sight picture, an adjustment to the sights needs to be made to change the angle. These sight changes move the position of the reticle inside the scope or the position of the sights. How much do you move the sights? MOA and mils are two different ways of measuring how much a sight movement will change the bullet impact.
A minute of angle is 1/60th of a degree. Think of it like this. There are 60 minutes in an hour. A minute is a sixtieth of an hour. A minute of angle is 1/60th of a degree, or a second of an hour. A full circle has 360 degrees. One MOA is 1/21,600 of the entire circle. Picture an analog clock. Now picture the area between 12 and 1. This is one hour, or sixty minutes. Now take that section and divide it into 60 equal pieces. Then take one of these 60 pieces, each representing one minute, and divide each by 60, representing the 60 seconds in a minute. This is how small 1 MOA is.
An MOA scope or sights using MOA will have “clicks,” typically half or quarter minutes. This means that each “click” or movement of the sights until you feel a slight catch, will move the bullet impact either a half minute for half minute sights, or quarter minute, for quarter minute sights. One MOA is equal to 1.047 inches at 100 yards. For general purposes, people generally round this to one inch. At 200 yards, one MOA is approximately equal to 2 inches, and so on and so forth. This means that for every one MOA you put on your sights, your bullet impact will move one inch for every 100 yards of distance you are shooting from, in the direction you move your sights.
It is easier for many people to think in terms of MOA because it seems simple, especially compared to an analog clock face. A milliradian is part of the metric system. Remember math class? There are 2π radians in a circle, approximately 6.2832 radians. There are 1,000 mils per radian and 6,283.2 mils in a circle. Comparing MOA to mils, 21,600 MOA in a circle divided by 6,283.2 mils in a circle yields 3.4377 MOA per mil. (Keep in mind all mil numbers are approximate because π is an irrational number.) One mil is a much larger measurement than one MOA, so it is generally broken into tenths for more precise adjustments. Another thought – mils are part of the metric system. 1/10 mil is approximately 0.9999 centimeters at 100 meters, larger than at 100 yards. (100 meters = 109.362 yards).
Remember that 1 MOA is 1.047 inches at 100 yards, though most round it down to 1 inch for easy math. Some call this rounded comparison “Shooter’s MOA.” The larger the distance, the more this rounding can be an issue. If you have sighters or unlimited shots, it is not a huge concern as you will be able to make a correction and will not be horribly off target. At 100 yards, 1 mil (3.477 MOA) is equal to 3.599 inches (3.4377 x 1.047). To convert MOA to mils, divide by 3.4377. To convert mils to MOA, multiply by 3.4377.
In short:
At 100 Yards:
1 MOA = 1.047 inches
1 mil = 3.6 inches
So which to use? The biggest preference comes from whether you prefer the standard or metric system. It is simpler for most Americans to think in MOA as everyone knows what an inch looks like and can picture minutes on a clock. Keep in mind though, that is because people tend to approximate 1 MOA to 1 inch at 100 yards, as distance increases rounding down can add up. For example, at 1,000 yards, 1 MOA is not 10 inches but 10.47 inches. For most shooters, however, 0.47 inch will not make much of a difference. Mils are not intuitive for standard system users. Most mil scopes have 1/10 mil clicks. One mil is approximately 3.6 inches at 100 yards. Divide by 10 and each click moves the impact 0.36 inches. Quarter minute MOA sights will move the impact approximately 0.26 inches, allowing for more precise changes. Both MOA and mils are accurate, the shooter just needs to understand the system to reach its true potential.