• Mil-Dot Reticles: History and Use

    With the growing popularity in the shooting world of Mil-Dot reticles in rifle scopes, I thought I would take a little time to explain what they are and how they work, as well as some differences in the commonly available models on the market today. The mil-dot reticle is primarily used as a range finder. The dots and spacing are of known sizes and ratios and can quickly be used to estimate the range to a known sized target.

    Not all Mil-Dot reticles are created equal, though. A word of warning to Mil-Dot users. The major optics companies offer up Mil-Dot reticles that are true (meaning the spacing and dots are of the correct ratios and sizes, respectively). I have seen a trend of less accurate spacing or dot size showing up in cheaper (particularly Chinese) optics. These are not mil-dot scopes, rather mil-dot style knock offs. As you will see, these substandard offerings are particularly worthless without figuring out the spacing and inventing new formulae for their use.

    What is a Mil-Dot?
    Contrary to popular belief, the Mil in mil-dot has absolutely nothing to do with the Military. It does not stand for Military and was not invented by the Military. The Mil is the common abbreviation for the milliradian (1/1000th of a radian). Whats a radian, you ask? A radian is simply described as a unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle. Huh? Look at figure 1. You see a pie shaped wedge as part of a circle. The three lines of this wedge are all the same. The arc (curved line along the circumference of the circle) is the same length as the two sub-tensions (the lines leading back to the center of the circle).

    Fig 1--A Radian (Rad) is the angle formed by the two sub tensions

    The radian has been around for a long time. The first person to use radian measurement was Roger Cotes, who worked with Isaac Newton in creating the Newton-Cotes Formula. The term Radian, however, is credited to James Thomson, brother of physicist Sir William Thomson (the Kelvin system of thermodynamics), who actually coined the word radian. Thomas Muir, a Scottish Mathematician and author published the term and its use as a unit of measurement in 1873 (over 160 years after Cotes came up with the concept). Now that Ive bored you with the history, lets turn theory into practice!

    How do they work?
    There is no particular magic to the use of mil-dot reticles. It is simply a matter of knowing the target size, bracketing it in your reticle to determine the mils, then plugging those numbers into a formula that will tell you the distance. Being a unit of measurement based on thousandths, this is pretty simple.

    The first step in the use of the milliradian is to figure out just how big it really is. A full circle contains 2p radians, so:

    2p rad = 360
    1 rad = (360)(2p) = (180)(p) 57.3

    The radian equals approximately 57.3. Knowing that there are 6283.2 mils in a full circle ( 2p/1000 ), we can determine that there are 3.44 minutes of angle in one milliradian. This will be important later when adjusting for the determined distance.

    Now that the useless math is out of the way, Ill let you in on the secret to the mil-dot reticle. The secret is 1000. Pretty simple, eh? Now you want to know how to use the secret, I suppose. I'm going to give you the secret, magical formula for range estimation using mil-dots:

    (TS x 1000)AM=D

    TS is target size and AM is apparent mils and D is distance to target. This formula relies on TS and D being equal units of measure (actually, it is based on meters, but works with any unit). Here is an example: a two meter target that scales as four mils on your reticle is how far? (2 x 1000)4=500 So its 500m away. A one yard target scales as six mils in your scope, how far is it? (1x1000)6=166.7 So this one would be 167 yards out. How about an eighteen inch target scaling 2.3 mils, but you want the distance in yards. This is where a calculator comes in handy! (.5x1000)2.3=217.4 You will notice that I converted the inches to yards before plugging into the formula. Remember that TS and D need to be the same unit for the formula to work.

    So what happens when you get a number that isn't easily converted to yards? Lets say you have a thirteen inch tall chuck in the scope that scales 2.1 mils. We will use a magic number (magic because for the life of me, I don't know where it came from, but it works), 27.7. (TSin x 27.7)AM=Dyd or in this case, (13 x 27.7)2.1=171.47 or 171 yards.

    How do I scale Mils?
    This can be a bit tricky. First, lets take a look at a mil-dot reticle. I am going to concentrate on the Army style (round) mil-dots as they are the most commonly available.

    Fig.2-1 -- Mil Dot ReticleFig.2-2 -- Mil Dot Reticle

    This is pretty standard, but I have seen dot sizes measure from 0.2 mils to 0.24 mils. The spacing is the most important thing, though. Basically, for the most accurate reading, you want to break the mils into tenths. Estimating your target size to the nearest tenth will give you your most accurate range estimation. Like most other forms of shooting, it all boils down to the nut behind the trigger. It doesn't matter if you know exactly how many minutes of dope to dial in for every distance you will shoot if you don't know the distance! Lets look at a few examples of scaling mils on a known sized target:

    Figure 3

    These figures represent scaling a target of known size to mils. The red circle is the target, can you guess the mils of each? It may be a little difficult on the computer here, so Ill help you along. 3-1 measures 1.4 mils, 3-2 measures 1 mil, 3-3 measures 3.9 mils and 3-4 is 2.8 mils. Since you put the target up, you know the size and can plug in the correct numbers to estimate range. Lets use figure 3-4 as an example. If we figure a thirty inch circle, then we get (30 x 27.7)2.8=296.8 or about 297 yards. If we figure the same circle as nine inches across, then (0.25 x 1000)2.8=89.28 or about 89 yards.

    How does this apply for game animals? Boy am I glad you asked! It is very hard to determine the size of a game animal without walking up and measuring him, so we rely on generalizations of game size. As an example here, Ill use the African Southern Impala. Generally, the impala measures twenty four inches from brisket to withers. In figure 4, you will see an impala thus bracketed for ranging: (24 x 27.7)3=221.6

    Figure 4

    Now that we have discovered range estimation as a primary function of the mil-dot reticle, well discuss a few ways of making it practical. Know your target! If, as in the example above, you know the size of the animal you are hunting (impala=24), then it may behoove you to make a cheat sheet or chart showing mils and distance based on the size. On the fly, you will know that 3 mils will be about 220 yards by looking at your cheat sheet. If you are zeroed for 200, hold dead-on and shoot. Quick and easy! Even quicker and easier is the use of the Mil-Dot Master ballistic calculator (see Jody Calhouns article on this handy product).

    What else can this reticle do?
    Holds for windage and elevations can be made using the mil-dot reticle as well. Knowing that one mil equals 3.44 moa, we can make fairly accurate hold overs or hold outs based on dot spacing. For windage, as an example, if we know that we need to hold out 7 moa right to compensate for wind, then the outside of the second dot from the center will roughly correspond. If we know that the estimated drop to our target from our zero is 10 minutes, then we know that two and a half dots will get us in the ballpark (in this case the kill zone).

    Hold outs for moving game can also be achieved. Using standard conversions for speed to minutes at your range, convert the hold out in the same manner as hold overs and hold outs for elevation and windage. Don't forget to add in (or take out) your normal windage holds as well! For example, if you are figuring for a 2.5 mil hold out for a left to right moving target and the target is moving in the same direction as a 2 mil wind, your hold out would be 4.5 mils. If the target is walking into the 2 mil wind (opposite directions) at the same 2.5 mil speed, then the hold out will only be 0.5 mil. It will take a lot of practice to hold for these corrections, but once you catch on, gizmos like laser range finders will become dead weight in your pack.

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