Long Range 101: Zeroing and Boresighting a Rifle
Zeroing is the process by which we establish a baseline point of impact for the weapon system from which we will make adjustments. It is rather straightforward but can be quite frustrating to those who are new or have a poor understanding of the theory behind it.
What we are trying to do is create a single, repeatable scope setting (windage and elevation clicks) that produces as close an impact to the center of the crosshairs as possible at a known distance. This is then followed by setting your scope’s turret markings to “0.0” (zero) for both windage and elevation, hence where the term comes from.
Let’s go over the process, and then discuss a few ideas about it.
The Process of Zeroing Your Weapon System - Mounting, Centering, and Boresighting
A key point is to try and achieve a solid zero in as few rounds as possible. Wasting a whole box of ammunition trying to find the target doesn’t do you as a shooter any good, and puts unnecessary wear on your barrel. We’ll start with a 100 yard zero.
Mount Your Scope Properly: First we’ll need to ensure that our scope is mounted properly. Follow the instructions carefully that came with your optic and rings, or, if you are very unsure, have an experienced shooter go over how it is done.
The most important things to ensure are: consistent, in-spec torque on your screws (both on the rings and base), proper eye relief to eliminate scope shadow and subsequent parallax issues, a properly leveled reticle, and pushing the rings forward in their notch on the base rail before torquing to spec.
There are whole articles written specifically on this topic about lapping scope rings, etc. but we will not get into that here.
Center Your Target In the Scope’s Crosshairs: The next step is to try to get the target roughly centered in the scope’s crosshairs.
Due to the inherent imprecision of some budget rifles, mostly in the placement of the scope mount holes in the receiver, simply mounting the scope and shooting at wherever the crosshairs happen to point after you’ve torqued everything down might result in some wildly-off center shots. To get an idea if this will be the case, we can use a technique known as Boresighting to try and get pretty close to our intended impact point, without ever firing a shot. Boresighting is an attempt to get the optic as closely aligned to the centerline of the bore as possible.
There are more than a few bore sighting tools out there involving lasers and rods placed down the barrel of the rifle, but the simplest and easiest method is absolutely free, requires no special tools, and produces arguably better results.
First, ensure the rifle is unloaded, and place the rifle in a stable but moveable position with the muzzle pointed at an easily identifiable, very definite target. Smaller is better. For example, the top edge or apex of a triangle or the corner of a square. On a bench with a deployed bipod and a rear sandbag works well.
Next, remove the bolt if your rifle is a bolt action. If your rifle is an AR, perform the previous step with the upper receiver only, and remove the BCG.
Then, look down the bore of your rifle from the rear of the receiver until you can see downrange to the target area. You may need to adjust or completely remove the stock in some cases. Center the target in the bore, (much like using iron sights) by moving the rifle until the target is as close to perfectly centered as you can make it, and adjust your sandbag or other stabilizing setup to where the rifle will stay in that position without being held.
Without touching the rifle, look through the optic and find the target.
Use the windage and elevation knobs ONLY to move the crosshairs to line up with the target as best you can. Check your adjustments frequently by continuously looking down the bore to ensure that the target is still centered. Remember: to adjust your POI up, rotate the elevation knob in the direction marked UP, and vice versa. The same principle applies to LEFT/RIGHT adjustments.
When you have the target centered in the bore as best as you can get, with the optic’s crosshairs also centered on the target as best as possible, replace the bolt.
That’s it. It’s easier than it sounds and should take less than 5 minutes. What’s more, with a little practice, you can achieve very good results and waste minimal amounts of ammunition. Boresighting tools, in the author’s opinion, are unnecessary and can introduce imprecise results if they are not of high quality.
The author fired this group as the first shots down a virgin barrel. The flyer at the top was the first round through the barrel after using the boresighting method described above, achieving a POI within 1.5” of POA. The following four shots were sighted off the first round.
The Process of Zeroing Your Weapon System - Test Firing and Adjusting Scope
Begin Firing Your Weapon: Once you have the rifle boresighted, you can begin the firing phase of zeroing. First, you will need to get into a repeatable position, usually, the prone. Remembering your fundamentals (Body position, NPA, Breathing, Trigger Control, follow through) and firearms safety, find and center your target in your crosshairs, load and fire a 3 round group to the best of your ability.
Observe the Impact on your Target: If your fundamentals are sound and your equipment is properly configured, you should see a tight group in the vicinity of your Point of Aim (POA).
Note the relation of the center of the group, which you can consider your point of impact (POI), to your POA. If your scope is equipped with an appropriate reticle such as MIL/MOA hash marks, it is best to note the angular distance between your POI and POA. If you are using a duplex hunting type reticle, you will have to convert the linear vertical and horizontal distance between your POI and POA into an appropriate adjustment.
Adjust Your Scope: Using your turret adjustments, adjust your scope’s windage and elevation to make your POI and POA converge. For example, if you are using a 100m zero and an optic with .10 MIL click increments, and your impact is 2.5Mils low and 1.3 mils right, you would rotate your elevation knob in the UP direction 25 clicks, and your windage in the LEFT direction 13 clicks.
As mentioned above, the windage and elevation adjustments are marked to represent a change in the POI, not the reticle. If you look at the reticle as you rotate the dial/turret, it will appear to move in the opposite direction to the printed arrow; i.e. rotating in the up direction will yield a downward movement of the crosshair. This is normal.
Reload and Refire Test Rounds: Load and fire another 3 round group using the same POA. Observe the POI and adjust as appropriate. Repeat until you have a tight POI group centered on your POA in the center of your reticle. With a bit of practice, you should be able to find and confirm your zero with a minimal amount of expended ammunition.
Zero Out Your Turret: If you have a scope with a turret zero setting feature, now is the time to “zero” your turrets. Consult your optic’s instruction manual on how to set the hash marks around your turrets to read “0.0” without moving the reticle.
Congratulations. You are now zeroed! Now let’s talk about a few things.
The Process of Zeroing Your Weapon System - Ideal Ranges
For most shooters, a 100 yard or 100 meter zero is ideal. Some shooters prefer to set their zero out to 200 or even 600 yards/meters in the case of large magnum calibers for dedicated extreme long range.
The truth is that it is mostly irrelevant. A modern rifle scope has a limited range of movement, or travel, in both the windage and elevation. This is limited mechanically by the internal workings of the scope itself, and the total amount of travel is set when the scope is mounted to the rifle.
For example, the scope I currently use has 19 MRAD (Mils) of elevation range. That means that if the elevation turret has 5 Mils of elevation per rotation, it has almost 4 full turns before it bottoms or tops out in either direction. When setting the zero to 100m, because I use a 20 MOA base, the 100m zero finds itself about the 2.5 mil point in the first rotation of the elevation knob. That means I have about 16.5 mils of possible elevation I can input into the scope to compensate for drop at extreme ranges.
If I were to set my scope up for a 200m zero, the scope itself doesn’t change its elevation at all. I’ve merely set the turrets to read “0.0” at whatever point in the total travel of the scope’s elevation results in a 200m point of impact, likely around 3.1 Mils from the bottom of the total travel for a .308 Win, leaving me with 15.9 Mils of maximum elevation travel to around 1150 meters, which does not change from the 100m zero.
Just because you set the zero out further does not mean you have somehow magically gained more travel in your scope, you’ve merely set your zero to a known DOPE at whatever range you choose.
Whatever range you choose to set your zero is a matter of personal preference and situationally dependent. Setting a zero close to your expected engagement range means that you might have to make a few less clicks to adjust for minor variances in the field. For example, if you are an NRA high power competition shooter who shoots at 600 or 1000 yards and has a dedicated rifle for that task, it makes sense to go ahead and set your turrets to zero at those ranges.
In some cases, the limits are practical. In the case of large magnum calibers dedicated to extreme range shooting whose max effective ranges dictate very large elevation ranges in their optics, they may be mounted to canted (built-in elevation) scope bases of 30 or even 40 MOA. Having elevation built into the mounting base on the rifle does actually give extra elevation, but it is impossible to “take out” that elevation for closer shots as the base is a solid piece of metal and obviously, not moveable. This means that the optic’s elevation will possibly bottom out before reaching an elevation to allow for a 100m or even 200m zero. In this case, the shooter will have to choose a zero that they can live with and simply “hold under” (i.e. aim below the target) for shots under that distance.
For hunters who prefer to use scopes without the ability to make on-the-fly adjustments, setting a zero is more about finding the “center” of an acceptable range of drop. A 200m zero for a hunter means that they can hold center on any target and achieve a hit somewhere within about .7-.8 mil above or below their POA from 0m to about 300m with a .308 Win for example. If that is acceptable to that hunter for an ethical harvest, so be it.
Zeroing is an essential part of precision shooting. It is a core task that must be mastered if you expect to be able to shoot with any confidence at all. Most competition shooters and many hunters choose to re-confirm their zero before starting their match or hunt, as conditions may drastically change and equipment is sometimes rough handled in transit.
Remember, your zero can change dramatically if you change any number of variables, from ammunition selection, to environmental conditions, to even a change in body position can significantly impact your POI. Range time, experimentation, experience and sound record-keeping are key to predicting how your rifle will behave in various circumstances.
Major Clutch is an active duty officer who shoots long range competitively in between deployments, overseas assignments, and helping raise his family of heathens. He is a huge nerd who spends an inordinate amount of time reading and re-loading. We at SOFLETE couldn't think of a better person to explain the science of marksmanship to our readers.