Mastering the MilDot

By David Day

Mil-dot reticle scopes were originally designed for use by the United States Marine Corps snipers and were used extensively in the Viet Nam war as a means of determining distance to long-range targets. The Mil in mid-dot stands for milliradian. As the intent of this article is to discuss use of the Mil-Dot scope in airguns and not confuse the reader with scientific jargon, lets leave it at that. If you want to learn more about the physics of how and why the mil-dot evolved as it did, fire up your computer and go to http://www.boomershoot.org/general/TruthMilDots.htm

The theory and practice of mil-dot range finding are simple. If one knows the approximate height of a target and the distance between two dots (center to center) on a mil-dot reticle at a given range and magnification, it is possible to determine, with great precision, how far one’s target is positioned downrange. From this information, sight holdover can be rapidly determined with precise shot placement at the estimated distance. That sounds like a real advantage for the sniper who has to deal with a bullet drop of several feet over a thousand yards or more! 

The standard mil-dot is designed so that the distance between any two dots (center to center) on the scope reticle at 1000 yards is 36 inches. If a standing human, approximately 72 inches tall (6 ft), is observed through a mil-dot scope at 1000 yards, and the crosshairs are placed at the very top of the person’s head, that person’s feet should just touch a position on the reticle that is at the center of 2 dots down on the vertical element of the reticle.

Knowing that the person is approximately 72” tall, and that one mil (the distance from dot center to dot center on the reticle) covers 36 inches at 1000 yards, one can deduce that a person subtending the a total distance between 3 dots is 1000 yards away (the counts the crosshairs as one dot center).

Now, let’s suppose we are again looking at a standing human through a mil-dot equipped riflescope. After placing the crosshairs at the top of the person’s head, it is observed that this person’s feet are 4 dots down from the crosshair of the reticule. Assuming we are looking at a person who is approximately 6 feet tall, we can deduce that the distance covered between any two dots at the range at which the person is being observed is 18 inches (72”/4 dots).
 
Again, knowing that the distance between any two dots at 1000 yards is 36 inches and that our reference point has indicated that the distance covered between two dots is 18 inches, simple algebraic math tell us that the distance to the target is 500 yards away.

X yds/18” = 1000 yds/36” 

X= (1000*18)/36 

X= 500 yards 

Mil-Dots and Airguns

The intended method of use for a mil-dot scope is all well and good if the target is several hundred yards away. But what about airguns, where the range is typically less than 100 yards?

The good news is, that as the range to a target decreases, the distance covered between two dots also proportionally decreases. At 100 yards, the distance covered between two dots on a mil-dot riflescope is only 3.6”, and at 50 yards, that distance is only 1.8 inches! If you happen to be lucky enough to be shooting an airgun that has a trajectory with a pellet drop of 1.8” from line of sight at 50 yards, simply align the first dot below the crosshairs on the target and you should be dead-on at 50 yards.

This utilization of the mil-dot technology for an airgun is a bit different that the way it was intended to be used in high-powered rifles at great distances. If you have an object of known size to shoot at, say a 2 inch diameter plastic ball or a 1.5” squirrel head, a mil-dot scope could be used as it was traditionally intended, but the math gets a bit cumbersome. And for every shot you make you do not want to go through the process as outlined above.

However, as long as you can fairly accurately estimate the distance to your target, either by experience or with the assistance of a range-finding device, the dots on your scope’s reticle can eliminate all significant estimation in hold over/under and totally free you of the need to change your scope’s elevation setting for the distance at which you are shooting.

All that this process requires is knowledge of your pellet’s trajectory and a little bit of understanding about the relationship between your target distance and how that translates into measurements on the mil-dot scale. Just keep in mind that at 100 yards, the distance between two dots (center to center) on the mil-dot reticle is 3.6 inches. Therefore the following relationships at various distances will be constant: 

Range to target distance between dots

So, if you are shooting at a target that is at 60 yards (distance between mil-dots = ~ 2.2”), and you know that your point of impact is roughly 4 inches below the point of aim at that distance, you would want to hold over on your point of aim by almost 2 dots so that the 2nd dot down from the crosshairs is just below the target.

An easier way to adapt an airgun to the mil-dot sighting method

Although the above seems straightforward, the example of a target at 60 yards with a 4 inch drop in point of impact still requires some mental calculations. This can be an impediment to timely acquisition of a target, determination of hold over and firing, especially when the target is a pest animal that is moving almost continually.

An easy way to deal with this is to make a ‘mil-dot map’ and place it on your scope so that it becomes a readily available reference to tell you where your point of aim should be at any given distance. Construction of a mil-dot map can be accomplished with just a pencil or a piece of paper, or image editing computer software can be used to develop a more professional-looking mil-dot map.

A mil-dot map turns an airgun into a highly efficient shooting tool without need for changing of scope settings 

A mil-dot map is nothing more than a graphical representation of the mil-dot reticle with marks that precisely identify the aiming point at any distance. When I construct a mil-dot map, I use image editing software and place ascending trajectory target marks for close-up targets on the left side of the map, and descending trajectory target marks for targets beyond the point of impact ‘zero’ on the right side of the map. I also add the type of pellet being used when the map was created to remind me to construct a new map if I should opt to change pellets in the future. Once completed, I apply two-sided tape to the underside of the map and affix it to the scope tube.
 
A mil-dot map for the Air Arms S410E with Kodiak match Pellets .177 cal
Although you can do the math manually to determine precise aiming points at various shooting distances on the mil-dot reticle, the Ballistic Solutions software available at www.silverstreaksports.com will automatically do this for you. After all required information is entered into the program to generate a pellet flight path, the program automatically creates a graphical representation of what your mil-dot map should look like. 

Although you can do the math manually to determine precise aiming points at various shooting distances on the mil-dot reticle, the Ballistic Solutions software available at www.silverstreaksports.com will automatically do this for you. After all required information is entered into the program to generate a pellet flight path, the program automatically creates a graphical representation of what your mil-dot map should look like. 

Ballistic Solutions Mil-Dot elevation plotting function

The mil-dot map makes application of the mil-dot scope to shooting at various distances a very precise, quick, and repeatable process. The first step in the process is to determine the target range. This can be done with a range finding device or the focusing objective on higher power scopes. As soon as you have determined the range to your target, refer to the appropriate aiming position on your mil-dot map. This will tell you which dot, or aiming point you should align to your target at the determined distance. Finally, acquire your target in the scope at the aiming point prescribed by the mil-dot map and fire!
 
It works for windage too!

Although the mil-dot reticle is primarily used as a device to determine appropriate elevation or hold over it has great application also in adjusting for windage. Most mil-dot scopes have dots on the horizontal as well as the vertical reticle element. If you have access to ballistic software that has a wind deflection feature, you can calculate windage deflection at any shooting distance for any given wind speed and plot it on the mil-dot map. This will allow for rapid determination of appropriate adjustments when shooting under windy conditions. 

A Mil-dot map for the Air Arms S410E with 5 mph wind deflection point of aim plots added 

Caution: Mil-dot scopes with variable magnification

For all but a few high-priced tactical mil-dot scopes, the distance between the reticle dots does not change as magnification is modified. It is important that you determine the magnification at which the distance between two dots on the scope’s reticle measures 3.6 inches at 100 yards. Usually the manufacturer will state this magnification in the documentation supplied with the scope. Some scopes will mark the magnification at which the distance between two dots measures 3.6 inches at 100 yards on the magnification adjustment ring.

If your adjustable magnification scope or its documentation does not indicate the appropriate magnification at which the reticle accurately measure 3.6 inches between dots at 100 yards, it can be easily determined with a simple test. On a piece of paper, make two marks, one above the other, that are 1.8” apart (center to center) and place the paper on a target holder at 50 yards (or 0.9” if you are going to do this calibration test at 25 yards). Then focus your scope at its lowest magnification and place the crosshairs on the top dot. Keeping the scope stationary, slowly increase the magnification until the first dot below the crosshairs is centered on the lower dot on your piece of paper. The magnification at which this occurs is the standard magnification for performing mil-dot calculations and creating a mil-dot map.

After you have created a mil-dot map at the standard magnification, determining appropriate hold over/under positions at different magnification is a simple process. If you are using a magnification that is ½ of your standard magnification, then the appropriate hold-over point at any distance is ½ of the distance from the crosshairs to the indicated point of hold on your mil-dot map. If you are using a magnification that is twice your standard magnification, then the appropriate hold over point at any distance is 2 times the distance from the crosshairs to the indicated point of hold on your mil-dot map.

One lesson learned relative to mil-dot scopes, especially some of the less expensive models, is that the actual magnification that results in the distance between two dots covering 3.6 inches at 100 yards is sometimes a bit different than the magnification indicated in the scope’s documentation. This is not a big problem. You simply need to perform the above test to determine the standard magnification for the scope you are using.

Why not just change the scope’s elevation to compensate for pellet drop? 

The most common way of dealing with the problem of a pellet’s rather pronounced trajectory compared to a firearm at short distances (other than just mentally estimating holdover) is to pre-determine how many clicks a scopes elevation adjustment knob will need to be turned to achieve a zero on the intersection of the reticle’s vertical and horizontal elements at usual shooting distances. Then, when presented with a target at a known or estimated distance in the field, one simply turns the elevation adjustment the required number of clicks to re-zero the gun at the intended target distance before firing.

Although this process seems simple enough, there are several issues associated with this method of re-zeroing a scope in the field.

The first, and probably most critical issue is correlation of the stated click value to actual change in the projectile’s point of impact at any given distance. Most scopes sold today are rated at 4 clicks per inch at 100 yards. This means that at a 100yard target, every change in the scope knob by 4 clicks should change the point of impact by 1 inch. However, if you put several identical scopes that are all rated at 4 clicks per inch at 100 yards in a firm support and test them against a known distance of measurement at 100 yards (say 10 inches), you will find that some of the scopes may require as little as 25 clicks (2.5 clicks per inch) to move the point of impact 10 inches at 100 yards, while others may require as many as 70 clicks (7 clicks per inch) to move the point of impact 10 inches at 100 yards.

This variability seems to be much more common in inexpensive scopes which tend to confirm the old adage ‘you get what you pay for’.
So, if you assume that your scope’s stated click value is correct without testing it prior to deciding how many clicks change are needed to re-zero your gun at any given shooting distance, you could very easily miss your intended target. This is especially true for airguns where the point of impact changes dramatically with only a few yards change in target distance at very close or relatively far away shots.
Another common problem with many scopes’ internal zeroing adjustments is that they often tend to ‘walk’ into their new zero after the scopes settings have changed. How often have you changed a scopes settings by, say 1 inch at 50 yards only to notice that the first shot after the change moved on the target by ½” while the second shot moved by ¾”, and finally shots 3-5 were on target? This can have a devastating impact on one’s ability to hit targets on the first shot if it happens to occur in the scope you are using.

The phenomenon of ‘walking’ is purely mechanical and is related to the ability of the internal erector tube’s tension springs to actually change the zero by a given amount when the scope knob is rotated by a modest amount. A source of the problem is often dried lubricants or over-lubrication on the adjustment mechanism. An improper lubrication condition tends to hold the tension springs in place in spite of modest elevation or windage knob adjustment. When the gun is fired for the first time after turning the scope knob, recoil and internal vibration forces the slack out of the adjustments that were created by adjusting the scope knob settings. Then when the gun is fired a second, or third time and the tension springs seat the erector tube into it’s new position, the visual effect is a series of shots that seemingly ‘walk’ into the intended point of impact. Shooting on a very cold day can also cause this problem in a scope that seems to function normally during warm summer months. Many knowledgeable shooters compensate for this mechanical limitation by twisting the scope’s windage and elevation knobs past the setting for the new intended point of impact and then reversing the direction of the scope knob by twisting back to the intended point of impact. This procedure creates more force on the tension springs and tends to take more slack out of adjustments causing them to more accurately seat the erector tube in its intended position before firing of the next shot.

A big advantage with a mil-dot scope is that the problems of ‘walking’ and accuracy of stated click values in scope knobs are entirely eliminated when the mil-dot method of sighting is used. Because one is not continually changing scope settings, the mil-dot sighting method tends to produce more shot to shot consistency in projectile placement. Also, because there is no need to re-zero the scope with changes in knob settings, shots with appropriate elevation can be fired more quickly using the mil-dot sighting method. This gives the mil-dot shooter an added advantage in hunting situations where the time from target acquisition to firing is critical to success.

Other related problems that are eliminated when the mil-dot method of sighting is used are human errors in scope knob adjustments. How often during a field target match has one heard that targets were missed because a scope knob was turned one full revolution the wrong way or one simply forgot turn the scope knob to it’s proper setting prior to making a shot. It happens more often than you would think and national class field target competitors have lost matches due to this human error. If it can happen to them, it can happen to any of us mortal shooters!

Mil-Dots for Air Pistols

Because of the pronounced trajectory of air pistols relative to air rifles and firearms, on would think that mil-dot scopes would be very popular for pistols. However, this is not the case. Of course, one could always mount a mil-dot rifle scope on an air pistol if comfortable with the close-up ‘taco’ hold. The one notable exception is the Burris line of pistol scopes with the ‘Ballistic Plex’ reticle. The reticle is not a full-fledged mil-dot. Rather it has a series of small hash marks on the bottom half of the vertical element of the reticle that function in the exact same manner as a mil-dot scope for determination of hold over at various distances. Experience with this reticle has demonstrated that it can extend precise shot placement with most air pistols out to 30 yards. This can turn many pistols into appropriate tools for pistol field target competition.

If you have never tried a mil-dot scope as a sighting device on an airgun, you will find it a pleasant experience. One caution is that most of the documentation that comes with mil-dot scopes is intended for high-powered rifle shooters at very long distances. Because the airgun shooter applies the mil-dot in a different manner than the long distance firearms shooter, the documentation can be unnecessarily confusing. Once you get past the unnecessary technical documentation, you will likely find mil-dot scopes to be both quick and easy to use and every bit as accurate as higher priced scopes that rely on continual scope zero setting changes for shooting at various distances.