Zeroing at a Fixed Distance with the Front Sight
Although the standard front sight on the 10/22 is fixed, some aftermarket sights, such as Tech-Sights (www.tech-sights.com) and Nodak Spud (http://nodakspud.com/NDS-22.htm), have height adjustment on the front sight post. This article will discuss how to adjust the front sight for zeroing, using the same scenario as in the last post on rear sight adjustment (Part 1). This time we are treating the rear sight as fixed and adjusting the front sight.
Below is a picture of the scenario. Click on it to see it full-size:
We still have two similar triangles. But this time the smaller triangle is inside the larger one, and both have the same end point at the rear sight. The distance AF is the amount of front sight adjustment needed to bring the point of impact (POI) down to the point of aim (POA).
Which way do we move the front sight to lower POI? We raise the front sight to lower POI. This way, on our next shot the muzzle will be lower when we align the sights on the target. The picture below explains why. Both barrels have the front and rear sights on exactly the same level and the rear sights are the same height. The red one has a high POI. After adjustment, the taller front sight on the blue barrel moves POI to the bullseye.
An easy way to remember which way to adjust which sight is FORS: “Front Opposite, Rear Same”. This is true for both elevation and windage. If you have fixed sights, like the 10/22’s standard sights, and you want to adjust the front sight for windage, you must move the front sight to the left to move the shot to the right.
Now, how far do we move the front sight? Let’s look at the math of these similar triangles:
AF/R = I/(D+R)
AF = I × R/(D+R)
Notice that the denominator is larger than in the rear sight adjustment, which was D alone. You can think of it this way: when adjusting the rear sight, the pivot point is the front sight; and when adjusting the front sight, the pivot point is the rear sight. The pictures also show that the amount of sight movement needed for a given change in POI is smaller for the front sight than for the rear sight.
Let’s look at zeroing using two popular aftermarket sight systems, both of which have adjustable front sights. We introduced Tech-Sights in our last article. The Nodak has a flip-type dual aperture rear sight and an adjustable front sight. It has some different features from Tech-Sights, and you might visit both websites to compare the two. The Nodak rear sight is not adjustable for elevation but is for windage. The key difference for this article is that the Nodak front sight post is an SKS/AK-style on a metric screw rather than the M16-type screw of the Tech-Sight, and the sight radius is .25” shorter.
In our scenario, we need to move POI down by 1 inch at a distance of 25 yards for our 10/22s with 18.5” and 22” barrels. Inserting these values into the equations we get:
- Tech-Sight, 18.5”: AF = 1” × 23/(900+23) = .0249” (vs. .0256” for the rear sight)
- Tech-Sight, 22.0”: AF = 1” × 26.5/(900+26.5) = .0286”
- Nodak , 18.5”: AF = 1” × 22.75/(900+22.75) = .0247”
- Nodak, 22.0”: AF = 1” × 26.25/(900+26.25) = .0283”
On both systems, we turn the sight clockwise to lower the sight (raise POI) and counter-clockwise to raise the sight (lower POI).
Now, how much to turn the sight on our 18.5″ barrel? The Tech-Sight uses the same 36 TPI screw on the front as on the rear, and the front sight post has the same 5 clicks per turn. It can be adjusted with an M16A1 sight tool. One full turn is .0278”, and four clicks are .0222”. If we make a full turn to raise the sight, we will lower POI by:
4 clicks: I = A × (D+R)/R = ‒.0278” × (923)/23 = ‒.891”
5 clicks: I = A × (D+R)/R = ‒.0278” × (923)/23 = ‒1.116”
So with four clicks we are .109″ high, and with one full turn now we are .116” low on the target. However, if we have a TS200, we can now adjust the rear sight, which would raise or lower POI by .0056” per click. Adjusting the rear sight by 2 clicks changes POI by .112”, which puts us just about in the center of the bullseye. Of course, if you can hold accuracy at 25 yards to within .1” without a magnified scope and a benchrest you’re a world-class shooter and should try out for the Olympics.
The Nodak sight post has an M6x.75 screw and requires an SKS/AK type sight tool. Each full turn is .75mm, or .0295”. The sight does not have click detents for fractional turns but is under spring pressure which acts like a lock washer, holding the post in position even under recoil. Not having clicks makes it precisely adjustable. If we divide the desired adjustment by the full turn movement, we’ll know how far to turn the screw – a little less than 7/8 of a turn:
.0247/.0295 = .8373 turn
The method presented so far is limited to a single distance, and requires that we do some basic math every time we have to adjust the sights. It introduced the basic concepts, but it isn’t optimal. In our next post we’ll talk about a more general approach to sight adjustment, which is the one actually used in practice.
Thanks to Tech-Sights and Nodak Spud for providing the technical specifications for their sights used in this article.