Need to know how to measure distances with your scope’s reticle? In this article I’ll show how to master this valuable long range shooting technique.
In the previous article about range estimation, I talked about the importance of measuring the distance between you and your target with the maximum degree of accuracy possible. I’ve listed some of the most used methods for range estimation, but my own preferred method is the one we’ll focus on in this post: using scope reticle to gauge range.
This technique has been used for decades by military snipers, and has been proven effective on battlefields, for artillery operations, since the 18th century. The theory behind it is the relationship between the angles of the reference marks on the reticle and the linear measurement of the target. Formally, it’s called triangulation. It is the same working principle of instruments like the theodolite and the sextant, used in surveying and navigation.
Using Mil Dot
The most commonly known of these “ranging” reticles is probably the mil-dot. Its name derives not from its military origin, but from the fact of being graduated in mil, which is, as we have seen (check out the full post here), is an angular unit of measurement. In a mil-dot reticle, the distance between the center of the reticle and the centre of the first dot, as well as de distance between the centres of two consecutive dots, is equal to 1 mil. The size of the dots varies among the various types of mil-dot reticles. In the standard mil-dot developed by the US Army, for example, each dot is equal to 0.22mil.
How to Triangulate
If you know the size of your marks and the size of your target, you can easily calculate the distance. All you have to do is to aim at the target, positioning the reticle to get the most accurate measurement possible. It is advisable to always measure the longest portion of target. In example, if your target is a silhouette, you should measure its height. This way, you will always maximize the accuracy of your triangulation.
Once you have taken the angular measure of the target, you can obtain the distance with a simple formula.
If you work with metric units of measurement, you just have to divide the size of the target (in millimetres) by the number of mil it measures on the reticle, and you will obtain the range in meters. For example, you know that your target is a paper sheet of 500mm X 500mm, aim at it with your scope and you’ll see that the target measures 1.5mil (from the centre of the reticle, to half way between the first and the second dot, in a standard mil-dot reticle). Calculate 500/1.5 and you’re answer is 333m, which is the distance from you to your target.
Working with US/Imperial units, the formula is a little different. You’ll need to multiply the size of the target (in inches) by 22.77, and then divide the result by the number of mils it measures on the reticle to figure out the distance in yards. For example, if you have a target of 20in x 20in, which measures 1mil on your reticle, calculate (20×22.77)/1 and you’ll 455yds.
Almost all the ranging reticles are graduated in mil, but there are also MOA reticles (I’m starting to see more of these on the market now than there used to be), that are more US measurement-friendly. If you have a MOA reticle, to calculate the range in yards, divide the size of the target (in inches) by the number of MOA measurements on the reticle, and then multiply the value by 100.
There are also formulas that allow you to mix units of measurement, but I suggest you to keep things as simple as possible and stick either with mil and metres/millimetres, or with MOA and yards/inches.
Target Field Measurement
As you can see, the formulas are pretty simple. What’s more challenging is to actually measure the target when you are on the field. Keeping the reticle steady enough for an accurate measurement is not always possible, unless you are using a bench rest. In addition, it’s not always easy to read the correct value at the decimal degree, and that’s that’s accuracy you need. If the target is small, or if it’s very far, a difference in reading of 0.1 or 0.2 mil can make a big difference. My advice is to always take multiple measurements, taking note of what it seems to be, at first glance, the correct value, and then take the average value between them. For example, if you are unsure if your target size between 0.6 and 0.8 mil, just take the average and input 0.7 into the formula.
An important thing to consider when you triangulate with the reticle is magnification. If the reticle is installed on the first focal plane, its size changes with the magnification (you see it becoming smaller or bigger), along with the size of the target, and you can use it at every magnification. On the other hand, if the reticle is on the second focal plane, varying the magnification, the reticule will always appear the same size, but it may vary in relation to the perceived size of the target. In this case, the ranging can be done only at one magnification. Ranging at different magnification, the angular value of the reticles would not be true anymore. For many scopes, the ranging magnification is the highest, but it is not unusual, especially on scopes with high values of magnifications, to find it half way. Always check in your scope’s specs at which magnification you can range, and always remember to set it correctly on the field before starting the ranging process.
But what if you don’t have a ranging reticle? Many scopes, especially hunting scopes, come with standard, non-graduated reticles. This doesn’t mean that you can’t use your scope to triangulate range. Many reticles are built following angular measurements. The scope I have on my deer rifle, for example, has a stanadard Plex reticle. At the range, I discovered that the thin part of the reticle measures 2mil in length from the center to the beginning of the fattest part of the reticle. It is not very accurate for ranging, but with some practice it can work, especially at short distances and when ranging with large enough targets.
There is also a trick that everyone can use for ranging. You can use the scope’s adjustment, instead of the reticle, to measure the target. If you keep the rifle steady while you rotate the scope’s turret, you can see the reticle moving. If you count the clicks it takes to move the reticle for the target’s entire height, you have the angular measurement of the target to insert into the formula. It is not simple, because you’ll need to be rock steady (repeat in your mind: “I am a stone. I don’t move” like in Enemy at The Gates) while you turn the turret. With practice, and maybe with the help of a “spotter,” this can serve as a very accurate and reliable method.
I suggest practicing these techniques as much as you can. Triangulate known distances, or confirm your estimations with more accurate instruments. You can triangulate distances even without going to the range. Triangulate the range of objects out of your windows for example (just mind the safety rules).
To test what you’ve learned from this article, you can try to estimate the distance the distance of the target you see in the featured image at the top of this page. The white paper target is 500mm X 500mm, while the reticle is graduated in mil (the big lines are 1mil, an the smaller ones measure half a mil). Give your best guess below in the comments.
In the next article, I’ll talk about how to determine, and compensate for, weather conditions that affect the POI: temperature, pressure and altitude.