RDS: Zeroing using MOA / MIL

This post discusses how to accurately zero in your RDS (Red Dot Sight) so the point of aim = point of impact.

 
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Every RDS has a windage and elevation adjustment to move the position the red dot of the RDS (i.e. the point of aim). When these adjustments are manipulated, you should perceive a tactile click. Each of these clicks represent a MOA (Minute Of Angle). The manufacture's specifications should indicate the distance each MOA represents (e.g. 1 MOA per click, 1/4 MOA per click, etc.)

Some windage/elevation adjustme indicate the MOA setting on the dial.

Some windage/elevation adjustme indicate the MOA setting on the dial.

What does MOA mean?

MOA means Minute Of Arc or Minute Of Angle, which is an angular measurement of a circle. A circle is 360°, and 60 minutes of arc in each degree, totaling 21,600 minutes of angle in a complete circle. Thus, 1 MOA is equal to 60 arcseconds, or 1/60 degree, or 1/21600 circle. The distance, or range, from the shooter to the target is also the radius [R] of the circle.  For any given range or radius [R], one minute is equal to 2πR÷21,600. Thus, if the range is 100 yards, that distance is equal to 3,600 inches (3 feet per yard, 12 inches per foot). When we plug 3,600 in for R in our formula, the result is (2)(3.1416)(3,600)÷21,600 = 1.0472 inches. Therefore, at 100 yards, 1 MOA is equal to 1.0472 inches. The formula can be multiplied for any range. If the range is 500 yards, 1 MOA equals 5.2360 inches. If the range is 10 yards, 1 MOA equals 0.10472 inches. However, when zeroing in their sight, many shooters equate 1 MOA t0 one inch at 100 yards, two inches at 200 yards, etc., since four hundredths of an inch is such a negligible adjustment that is impossible of being achieved via the elevation/windage dial.

What is a MIL?

A MIL relates to the U.S. military variation of a unit of angle known as a milliradian. The distance between the centers of any two adjacent dots on a MIL-Dot reticle scope equals 1 Mil, which is about 36” (or 1 yard) @ 1000 yards (i.e. 3.6 inches at 100 yards).

Adjusting RDS to Point of Impact

First, shoot a 3- to 5-shot group at the target from 10 yards away (25 yards for a rifle). Assuming you are not a machine, the bullets will be proximate to each other. If a round is disproportionately further than the others, do not consider thie as part of your group—it is a 'thrown round' that is the result of human error.

Once you shoot your shot group, make a 'best guess' estimate as to where the center of your group is (i.e. point of impact), and measure the center of the group vertically and horizontally (in inches) to the center of the target (i.e. point of aim). Recall above, 1 MOA is equivalent to .10472 inches on the target at 10 yards. Divide the the distance by .10472 to determine how many clicks you would need to adjust the red dot. When adjusting the RDS, you are moving the bullet to the point of aim, NOT the red dot to the point of impact. For example, if at 10 yards your bullet is 1.2 inches low and 1.4 inches to the left, you would move the elevation UP 11 clicks (1.2 ÷ .10472 = 11.45) and windage 13 clicks (1.4 ÷ .10472 = 13.36) to the RIGHT.

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In the example to the right, three bullets struck low-left of the bullseye. The red X identifies the center of the group that should be used when calculating sight adjustments. This particular target is specifically designed for zeroing your RDS—it shows each MOA necessary for proper sight adjustment. Here, the elevation needs to be adjusted UP one click, and windage adjusted to the RIGHT two clicks.

Below is a short list of other calculations for different MOA sizes and target distances:

  • 1/4 MOA per click

    • 1/4th of an inch @ 100 yards

    • 3/16th of an inch @ 75 yards

    • 1/8th of an inch @ 50 yards

    • 1/16th of an inch @ 25 yards

  • 1/3 MOA per click

    • 1/3rd of an inch @ 100 yards

    • 3/12th of an inch @ 75 yards

    • 1/6th of an inch @ 50 yards

    • 1/12th of an inch @ 25 yards

  • 1/2 MOA per click

    • 1/2 of an inch @ 100 yards

    • 3/8th of an inch @ 75 yards

    • 1/4th of an inch @ 50 yards

    • 1/8th of an inch @ 25 yards

  • 1 MOA per click

    • 1 inch @ 100 yards

    • 3/4th of an inch @ 75 yards

    • 1/2 of an inch @ 50 yards

    • 1/4th of an inch @ 25 yards

Below is a 1 MOA per click / MIL zeroing chart using the firm 1 MOA = 1.0472 inches for one to 200 yards:

Click on image to enlarge