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How to Zero a Rifle Scope (MOA and Mil)

Posted by RangeOften.com on Oct 1st 2020

How to Zero a Rifle Scope (MOA and Mil)

We've covered some of the best practices of good shooting,  including the fundamentals of marksmanship. But before you lay out your gear at the range and start trying to put rounds on target, you've got to get your guns zeroed. Today, we're covering how to zero a rifle scope.


What is Zeroing?

When you buy sights for your gun, you can't just throw them on, aim and shoot, and expect a bullseye. Zeroing means adjusting the sights so your  point of aim (POA) matches your weapon's point of impact (POI) at a given distance. That is, there will be zero difference between the two points. Whether you're zeroing a scope, red dot, or iron sights, the process is the same.

How does it work?

You need to first determine where rounds are impacting (POI) based on POA. The best way to achieve this is by aiming at the center of a large paper target at a known distance, pulling the trigger, and recording where the round landed. If the round landed low and left of where you aimed, you need to adjust the sights lower and left. This adjusts point of aim down and left to match the bullet's impact. Basically, you want the reticle to wind up on the exact spot where your round landed in the first place. The process is more complicated than this but we're going to walk through each step with visual aids.

Example

Reticles use angular measurements to calculate the difference between POA and POI. As distance increases, adjusting POA will make a greater difference in POI than if the same adjustment is made at a closer distance. As an example, if you adjust your reticle so point of aim shifts, say 1" left at 100 yards, then it will also shift left 10" at 1,000 yards. A good way to visualize how this works is to imagine yourself shooting inside a large circle that runs over top of you, behind you, and in front of you.

The size of the circle is determined by the distance to the target. POA is the red line. POI is the orange line. If we measure the width of the angle created by these two lines, we get the distance between the point of aim and point of the bullet's impact. As distance to the target increases (as the circle gets larger), the angle's width increases proportionately. In our example, our target was 100 yards away and there was a 2" difference between POA and POI. If we increase the distance to our target from 100 yards to 300 yards but make no adjustments to our sights, the distance between POA and POI increases from 2" to 6". That's how much our angle's width increases.

Real-world factors like bullet drop, elevation, and wind will affect this angle. But this is the mathematical way a reticle measures bullet impact against aim. This is also why it's important to zero your scope at a known distance: The reticle's point of aim is adjusted with fixed increments (0.25" per click at 100 yards, for example) so you won't know how many clicks to make on your reticle unless you know the distance to your target.


Types of Reticles and Turrets

These MRAD turrets provides 0.1 mils of adjustment per click.

So, how do we actually measure these angular differences in point of aim and impact if we know our distance to the target? We need a measuring tool and that tool can come in the form of the reticle itself. Your scope's reticle can measure the distance between POA and POI with either a series of horizontal and vertical dots or lines, or a grid-like pattern that overlays the target. These are all called subtensions, or subtended reticles. The turrets are the exposed knobs on the body of the scope, and they control the vertical (distance/elevation) and horizontal position (windage) of the reticle.

Mil/MRAD (Milliradian)

Milliradians, or simply mils or MRADs, divide our imaginary circles into 6,280 equal parts. One mil equals 3.6" of adjustment at 100 yards. If you enjoy the metric system, 1 mil equals 1 centimeter at 100 meters, 2cm at 200 meters, and so on. If we used mils in our example above, we would need to adjust POA about 0.5 mils up to close our 2" gap to the bullet's impact.

MOA (Minute of Angle)

Minute of angle divides our circles into 21,600 equal parts.  One MOA equals 1" of adjustment at 100 yards. Conveniently, one MOA is also 0.25" at 25 yards, 2" at 200 yards, 3" at 300 yards, and so on. In our example, we would need to adjust our aim upward by 2 MOA to close our 2" gap at 100 yards. 

Which Reticle Should I Pick?

MOA is the more popular reticle and turret combo for most shooters, simply because it's a lot easier to use. Calculating your ballistic data in mils is more difficult if you're used to the Imperial system of measurements. Basically, if you're American then you enjoy inches, feet, and yards more than centimeters and meters (we do, too) and the mil system is designed for calculating adjustments based on the metric system.


How to Zero a Rifle Scope

Now we understand point of aim and point of impact, and how distance affects both. We also know how to calculate the angular measurement between these two numbers. Let's see how to use the subtensions and turrets on our scope together, so we can convert that angular measurement into scope adjustments. 

Adjusting the Turrets

All scopes adjust the position of the reticle (POA) with turrets. Many scopes have three: One for windage adjustment (left/right), one for elevation and distance (up/down) and one for focus. You will only use the first two turrets to zero, the latter is for achieving clarity. Refer to your scope's instruction manual to determine how many clicks of each turret is equal to one MOA or MRAD. Most MOA turrets use 1/4-MOA adjustments, meaning each click moves point of impact 0.25" at 100 yards. Most MRAD turrets use 1/10-MRAD adjustment, or 0.1" at 100 meters.

MOA Reticle @ 100 Yards

This scope's reticle has one-MOA tick marks. Our round landed 3 MOA right and 2 MOA high. At 100 yards, that's an easy translation: We need to adjust our scope 3" right and 2" up. On a quarter-click (four clicks per MOA) turret, that translates into 12 clicks right and 8 clicks up.

MOA Reticle @ 25 Yards

At 25 yards, one MOA is 0.25". The round landed about 5 MOA (1.25") low and 2 MOA (0.50") left. With 0.25-MOA turrets, that translates into 20 clicks down and 8 clicks left to zero POA with POI.

MRAD Reticle @ 100 meters

MRAD scopes use reticles with subtensions that look identical to MOA subtensions. In this case, they're called "mil dots." Our round landed 4 mils right and 2.5 mils low. At 100 meters that translates into 14.4" rights and 9" low. With 0.1-mil turrets, you would need to adjust right 160 clicks and down 100 clicks.

MRAD Reticle @ 25 meters

Using our mil dots to subtend, we see the round landed 4 mils left and 4 mils high. At 25 meters, this translates into 3.6" left and 3.6" high. With 0.1-mil turrets, that translates into 40 clicks left and up to zero POA with POI. 


Zeroing Without a Reticle

Not all scopes have mil dots or MOA subtensions, and iron sights and red dots obviously don't either. So, how do you zero your rifle or pistol's other, not-so-fancy gunsights? It all depends on the targets you're shooting at. In this case, we replace our subtensions and dots with physical grid squares and markings on the target itself.

Depending on how your iron sights or optics adjust (mils or MOA), you'll need to select a Rifle Sighting Target with either 1" or 1cm grid squares. Simply plant your target at a known distance -- again, likely 25 yards or 100 yards -- and start getting rounds on target. Instead of using a reticle to measure the distance between POA and POI, the grid squares on the target measure for you. Aim center-mass, and figure out how many grid squares off-center your round landed. Then you can calculate how much you need to click your front or rear iron sights or red dot in the appropriate up/down and left/right directions. 


Zeroing by Bore Sight

If you're struggling to put rounds on paper or you don't have a fancy reticle or Sighting Targets, there is still a third way to get your POA and POI matched up. Boresighting is the practice of aligning the scope, optic, or gunsight with the bore by literal sight .

Naked Eye Boresighting

How does it work? Boresighting with the naked eye is easy: Pull the bolt out of your rifle's chamber and look through the receiver so you can see through the barrel and downrange. Align the barrel with a fixed point of reference at a distance of about 25 to 50 yards. The smaller and farther away the object, the better. Then you can align the optic with that same object, which should bring POA close to POI. This will not get you an effective zero, but it will help you get rounds on target if your POA is way off.

Laser Boresight

Laser boresights can get an effective zero up to 100 yards away without ever firing a live round. The laser boresight uses a pen- or cartridge-shaped laser which sits inside the chamber or at the muzzle's opening. The laser replicates point of impact of a live round with a red or green beam, which provides a point of reference to align your optic. 

Boresighting is great because it doesn't even require calculations. You don't need subtensions or grids on a target. Just adjust the reticle until it matches the position of the laser beam, and you're ready to load a magazine and send some rounds downrange. You may need to fine-tune your final zero after boresighting, because the position of the laser can vary slightly from the exact position of the bore.


The Best Distance to Zero

(Spoilers, it's 25 yards)

Picking what distance to zero your rifle at can be difficult because different distances affect the overall arch of the bullet as it travels. The goal is to pick a zero distance that affords the most "point-black" shooting across multiple distances. That is, firing at a set distance will result in POA perfectly (or almost perfectly) matching POI without any on-the-fly adjustments.

By zeroing at 25 yards, the bullet trajectory for most .22- and .30-caliber centerfire rifle cartridges remains relatively flat for a substantial distance: Your rounds will typically land just 1.5" to 2.5" high at 100 yards, and POA will nearly match POI (less than 1" difference) at 200 yards. This is a perfect bullet trajectory for personal defense, close-quarters shooting, recreational range time, and hunting. Most indoor and pistol shooting ranges are 25 yards, too. It's convenient to pick this zero because it can be used for all your long guns and handguns.


Recap

Now you should have a basic understanding of why it's important to zero your rifle scope and other gunsights, and how it works. Here's a quick cheat sheet with the most important info:

  • Zeroing is adjusting point of aim (POA) so it matches point of impact (POI) at a known distance.
  • Two ballistic data systems are available: Minute of angle (MOA), and Milliradians (MRAD/mils).
  • MOA and MRADs/mils calculate POA vs. POI with equidistant reticle marks called subtensions.
  • 1 minute of angle subtension equals 1" at 100 yards, 2" at 200 yards, and so on.
  • 1 mil subtension equals 1cm at 100 meters, 2cm at 200 meters yards, and so on.
  • Most MOA turrets provide 1/4-MOA adjustments.
  • Most Mil/MRAD turrets provide 0.1-mil adjustments.
  • The best (and easiest) distance to zero at is at 25 yards.
  • If your sights don't have subtensions, buy Sighting Targets.
  • Targets come with either 1" or 1cm grid squares for zeroing.

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