Zeroing at a Fixed Distance with the Rear Sight
Questions always come up about zeroing the sights or scope, so this begins an irregular series of posts on the topic. We’ll start with the simplest concept of zeroing based on similar triangles, and eventually cover minutes of angle, adjusting for bullet drop at different distances, point-blank range (aka battle-sight zero) and the math underlying .22LR ballistics and bullet trajectories.
These topics are covered in books and all over the internet, with varying degrees of clarity, assumption of prior knowledge, mathematical integrity, and applicability to the real world. I’ll do my best to be as clear as possible, to be faithful to the correct math, and to apply the concepts to our 10/22s and their most popular sighting systems.
Let’s start with some basic concepts:
Point of Aim (POA): the spot on the target where your eye is pointed when you align the sights.
Point of Impact (POI): the spot where the bullet actually hits.
Sight Radius: the distance between the front and rear sights.
Here is the scenario: You are shooting at a target 25 yards from your front sight. Your bullet’s point of impact is 1.00 inch above your point of aim. You want to adjust the sights so that the POI and POA are the same. Since your POA is right where you want it – the center of the bullseye – and the shots landed high, you want to move the POI down by 1.00 inch. The principles and calculations below apply to any error of either elevation (up-down) or windage (left-right).
Depending on how your sights are configured, you can adjust the rear, the front, or both. The 10/22’s standard sights have an adjustable rear and a fixed front, so we will look first at adjusting the rear sight. Now the question is, which direction do you move the sight, and how far?
The picture below shows the elements of our problem:
The green line is your line of sight. The red is the line from the muzzle to the POI. These two lines, plus the vertical line on the target that connects them, make up a triangle. If we extend the red line to the rear sight, now we have two triangles that mathematically are called “similar” – meaning they have the same angles, and the lengths of their sides are in the same proportion.
(Note for the physics purists: since everything is at the same distance, we are ignoring the curved trajectory of the bullet.)
We want to adjust the rear sight so that the green line rotates to cover the red line; i.e., to make POA and POI the same. Then when we take our next shot, we will have the POA/POI on the target. We adjust the rear sight down to move the POI down. When we aim our next shot, the muzzle will be lower in order for the front sight to be aligned with the newly lowered rear sight.
How far do we adjust the rear sight? Because the triangles are similar, we can calculate the amount that the rear sight has to move. The ratio of rear sight adjustment (A) to the sight radius (R) is equal to the ratio of the POI-POA difference (I) to the distance from the front sight to the target (D). Here is the equation from the picture above:
A/R = I/D
A = I × R/D
Now you have to know the sight radius of your 10/22. On two of my 10/22s:
- the OEM sight radius with 18.5” barrel is 15.0”
- the OEM sight radius with 22.0” barrel is 18.5”
- the Tech-Sights radius with 18.5” barrel is 23.0”
- The Tech-Sights radius with 22.0” barrel is 26.5”
(Note: Both barrels were shortened by about 1/8” when they were re-chambered. You should measure yours, though 1/8” isn’t much difference.)
So the amount of adjustment would be:
18.5” barrel OEM sights : A = 1” × 15/900 = .0167”
22.0” barrel OEM sights: A = 1” × 18.5/900 = .0206”
18.5” barrel Tech-Sights: A = 1” × 23/900 = .0256”
22.0” barrel Tech-Sights: A = 1” × 26.5/900 = .0294”
Note that with the longer sight radius, it takes more sight movement to get the same POI change. Put another way, for a given amount of sight movement, the effect at the target is smaller. This means that sights on longer barrels can be adjusted (and aimed) more precisely than shorter ones.
Now we can physically adjust the rear sight on our 18.5″ 10/22 carbine. On the OEM rear sight, loosen the retaining screw on the right side of the leaf (the outer part), and move the blade down in its slot in the leaf. A problem is that the measurement scale on the sight is too coarse to make a precise adjustment, so it will be a trial-and-error process. It is hard to judge 1/60” (.0167”) by eye. You could get close using a caliper or depth gauge to measure the distance between the tops of the blade and the leaf. The Ruger Instruction Manual says each mark on the sight moves POI by 1.00” at 25 yards, but I haven’t tested that.
A popular aperture sight upgrade for 10/22s is the Tech-Sights system. The Tech-Sights TSR 100 sight has two apertures that flip to alternate. For a 10/22 with an 18.5” barrel, the POI elevation difference between the long-range and short-range apertures is about 1.5” at 25 yards. You can make finer adjustments by adjusting the front sight, which we will cover in our next article. Windage is adjustable just as on the TSR 200 sight.
The Tech-Sights TSR 200 rear sight allows reasonably precise, repeatable adjustment for elevation and windage, with a spring-loaded detent to hold the aperture in place. To lower the POI you turn the dial counter-clockwise; to raise POI turn it clockwise. The rear sight post is threaded at 36 threads per inch, and each click of the detent is ⅕ of a turn. So for each full turn the sight moves by .0278”; for each click it moves by .0056”. Four clicks would lower the sight by .0222”; a full turn of .0278” is closer to the desired adjustment of .0256″.
If we do a full turn on the rear sight, what will our error be? Take the difference between the desired movement and the actual, and multiply by the distance/sight radius:
I = A × D/R = (.0256 ‒ .0278) × 900/23 = ‒.0861”
The POI would now be .0861” below the center of the Bullseye. If that is too much error for you, you need to spend a lot more money on a precision sighting system or high-quality scope. The sights used on .22 rifles in international competition, such as the Olympics and ISSF World Cup, have adjustments as fine as 1 millimeter at 50 yards – but they cost hundreds of dollars.
If we take a full turn on the Tech-Sight on the 22″ rifle, our next shot will be .0543″ high:
I = A × D/R = (.0294 ‒ .0278) × 900/26.5 = .0543”
The easiest way to adjust the Tech-Sight is with their TS211 tool:
For elevation, use the crescent-shaped part. Notice that one prong has a tooth – this is the end you want to put pressure on so that it catches in the notch of the dial to push the dial over the detent.
For windage, use the flat, two-toothed side of the tool on the dial at the right side of the sight. Insert both teeth and put pressure on the tooth that is engaging the detent post at the bottom of the dial, and press the post fully to rotate the dial. The windage screw is also 36 TPI with five clicks per turn, so the movement per turn and per click is the same as shown above for elevation. Turn the dial clockwise to move the POI to the right; and counter-clockwise to move POI to the left.
Thanks to Tech-Sights (www.tech-sights.com) for providing the technical specifications for their sights used in this article.