Home » Tactical and Firearms » Range Finding, Mil Dot, DuPlex (Plex) Reticle, Parallax, MOA.

Range Finding, Mil Dot, DuPlex (Plex) Reticle, Parallax, MOA.

mil dot reticle
Mil Dot Reticle

I’m working on understanding scope adjustments and use. I had always thought mil dot stood for Military Dot Reticle System. Nope, it means Milli-Radians. A radian is PI(3.14) at 180 and 2 PI(6.28) at 360 degrees. Milli-Radian is 1/1000th of a radian. Also just a note, this means there are 6.28 radius (one radian is a radius) of a given circle around its circumference. But what this means for us is that the image in the scope at 10x power is 1/1,000th the actually size of the object. So a 1 meter tall target at 1,000 meters is 1 mil dot tall. A 1 yard tall target at 1,000 yards is 1 mil dot tall. A 1 foot tall target at 1,000 feet is 1 mil dot tall. A 1″ target at 1,000″ is 1 mil dot tall. A 1 cm target at 1,000 cm(10 meters) is 1 mil dot tall.

Remember that is at 10 power. So 5x power and the target is now 1/2 mil dot tall. 4x power and it’s slightly less than 1/2 mil dot tall. A Mil dot (10x) at 100 yards is 3.6″ (that is 1/10th of 1 yd(36″) at 1,000 yds) There is nearly 17 Mil dots in 1 degree and 1 degree is 60 MOA. (scope adjustments are in fractions of  Minute Of Angle) Or there is .28 or close to 1/3rd mil dots in one MOA. Saying it another way, there are 3 MOA in one Mil Dot. And one MOA at 100 yds is close to 1″. 200 yds, 2″ and so on. So you can see that, once you get this straight in your head, you can really fine tune your POI (point of impact).

Could a mil dot reticle be used for short ranges such as air gun ranges? Sure. A table could be made to help you with this. I’m not totally convinced that my math is correct here. So check me on this. I made a spreadsheet as such. Mil Dot is 1/range/(%of10x*1,000). % of 10x is power /10.  In this case, 4 power scope.

foot mil dot range ft x power % of 10x
1 20.0 20 4 0.4
1 13.3 30 4 0.4
1 10.0 40 4 0.4
1 8.0 50 4 0.4
1 6.7 60 4 0.4
1 5.7 70 4 0.4
1 5.0 80 4 0.4
1 4.4 90 4 0.4
1 4.0 100 4 0.4
1 3.6 110 4 0.4
1 3.3 120 4 0.4
1 3.1 130 4 0.4
1 2.9 140 4 0.4
1 2.7 150 4 0.4

MOA is Minute of Angle or 1″ at 100 yards. Actually 1.047″ at 100 yards. My new BSA scope has 1/4 MOA adjustment for both windage and elevation. This means 1/4″ at 100 yds. So this would be 1/8″ at 50 yds or 1/16″ at 25 yds. (This scope adjustment has nothing to do with the power of the scope)  One feature of this BSA scope I really like is the metal screw-in lens cover end caps. I have decided to zero my Chinese air gun at 20 yds because it is shooting at only 420 fps. This gives me a kill zone from 4 yds (12 ft) to 22 yds (66ft) of 2″ diameter. What I need to do is buy a new air gun with higher fps or repair this one.

Once zero is set it can be moved with the elevation adjustment. At 20 yds 1″ is about 20 clicks. A person could also re-zero at 25 yds, 30 yds and 15 yds noting how many clicks is needed to reach each zero point. However the ballistics and kill zone changes dramatically when you change the zero point. You really need to know at each 0 point how high the pellet will rise above the line of sight. Will also need to know where it drops a few inches below line of sight. You can use ballistics software to determine this or simply do some shooting of targets at different distances.

plex reticle
DuPlex Reticle

Above is a common reticle which can be used for range finding. This is the reticle on my BSA 4×32 scope. I have yet to find out specs for it such as MOA of the thick to thin line transition you see in the image. Or how many inches that is at 100 yds from left to right or up to down. Many of these scopes are 30″x30″ at 100 yds. So, with lack of proper information, I may have to use empirical methods for figuring this out with my particular scope by sighting through the scope at a target at 100 yards.  This process should be simple enough. I will set up something with 1″ or 2″ marks on it at 100 yds from viewing position. Then count the number of marks from center to left or whatever. Double the results to get size of object in inner box.

Let’s say the inner crosshair box is indeed 30″x30″ at 100 yds, that means a coyote at 100 yards would nearly fill the box. A crow 8″ tall would be half the distance from outer box to center of cross hair. If the crow is at 25 yds, then it would fill the box top to bottom. It’s a matter of ratios. It’s also a matter of knowing the average sizes of the game you are hunting. Also, one can range objects such as fence post, pop can, bottle or other refuse. Just about anything you already know the size of in the field of view. Then, when game moves into that position, you have a jump start on estimating its range.

Using Parallax Adjustment to Determine Range

I think this blog link can explain better than I, so here it is     “Parallax and Scopes”. This BSA Air Gun Scope I bought has a parallax adjustment on the objective bell up front. It shows 7.5 yds to 100 yds to infinity. It has markings adjustments on it for ranges between 7.5 and 100 yds. As that article suggests, they probably are not accurate. Also the article says that on lower powers, such as this 4 power scope, and at closer ranges, such as air gun ranges, parallax is not a huge problem. This may mean it will be more difficult for me in this situation to use this adjustment in ranging targets.

Basically if you change your gaze or focal point forward and back, this will cause some movement to appear in the reticle between the target and the crosshairs. If you notice this movement, you have parallax and then can adjust the setting until there is no parallax. The markings on the bell adjuster will give you the range. Conversely, if you know the range by some other method, then you can reach up and set it to that range and know that most of the parallax is removed. Also, that article said that parallax has nothing to do with focus adjustments. My scope comes with a focus adjustment on the front.

For air guns some good free ballistics software can be found at Hawke ChairGun Pro

As we can see from the chart above, we have a 2″dia. kill zone from 3 yds to 25 yds. The pellet will rise to roughly 1/2″ above line of sight at 13 to 14 yds. It also drops to 2″ below line of sight at 28 yds. Keeping this in mind after you have ranged your target (game) would be important.

If anyone wants to clarify or even correct any of this in comments, please feel free.

I purchased a Chinese Spring Piston .177 cal Air Rifle for $40 and he threw in a box of 250 pellets. New, this gun goes for about $80. I owned one of these once almost 15 years ago. Currently I want this gun as a “hunt near the BOV” gun, as it’s a bit heavy for packing. Back then I wanted it for target practice because I had been scoring only marksman on military M16 qualifications around 23 of 40 shots. I practiced my marksmanship fundamentals with this pellet gun in the backyard for a few months just prior to the next qualification. That next time I scored sharp shooter, which I think was a score of 35 of 40 shots.  That was almost expert, which was 36 to 40 of 40 shots.

I wanted this gun in particular because it comes with front adjusting sights and rear adjusting sights for both windage and elevation. On this gun I just purchased it appears that both front and rear sights adjust for windage and elevation, though I know for sure the rear sight adjust that way. The rear sight also had 8 quick settings for varying distances which adjust its height.

The ballistics on a .177 varies quite a lot from the high powered .223 at 2,500 fps. I believe the .177 exits the muzzle at 900 to 1200 fps on most air guns of this caliber. I chronographed this gun and it shoots at 420fps. I’m not totally sure that it may not need servicing, which might increase the fps. It doesn’t matter how many fps the gun shoots, marksmanship fundamentals remain the same. For rifle this is Position, Aim, Breathing and Trigger Squeeze. For pistols it is Stance, Grip, Aim and Trigger Squeeze.

The above clickable image is an Excel file that I wanted to share. This chart shows a silhouette size for targets 6′ tall (human size) for varying distances. 25 up to 500 yards. It gives you meters for comparison sake. It shows you in yards, feet and inches how tall and wide the target silhouette must be in order to simulate a sighting view size for targets at various distances. The only value you change on this is the yards to the real target. This example is set to 20 yards. So we see that, at 20 yards, your silhouette must be 5 12/16 tall by 2 14/16 wide to simulate a human at a distance of 250 yards. In other words, in your sight view, that silhouette will appear to be the same size as a human at 250 yards.

This is good, it gives us a way to practice for longer distances even though we only have 20 yards of back yard to shoot in. Of course the ballistics are not the same for higher powered rifles at longer distances, but we still get to practice all the fundamentals which include aiming and sighting.

On the spreadsheet above, you may however set that distance to 10 yards, 20 yard, 25 yards, 30 yards, 50 yards or whatever you like and it will give you appropriate silhouette sizes. How did I come up with the formulas? Ever hear of Angular Size Calculations? Angular Size Calculator And they have a formula (Angular size in degrees = (size * 57.29) / simulated distance). This is in column C on the spreadsheet. I then have to divide that by 2 (in other columns we multiply back by 2 to get full size) to figure 1/2 the tallness of the silhouette because the next math deals with trigonometry of right triangles. The formula is (Opposite side=tangent(angle)*Adjacent Side) which is in column E. Adjacent side in this case is the simulated distance. I believe I also had to covert degrees to radians in the formula as well with a spreadsheet function radians().

I made this for simulating rifle distances(ranges). However there is no reason it could not be made to work for pistol distances as well. So that you could emulate a pistol shot at 10 yards, 15 yards, 20 yards, 25 yards, 35 yards, up to whatever, 50 yards, 100 yards. Also It could be changed from 2 yard tall target to a smaller animal sized target other than a human, such as a rabbit or hog.I did this on page 2 and page 3 in the worksheet download above. Instead of 2 yards change the formulas to use 2/3 yard or 1/3 yard. Though the height to width ratio would be inverted meaning the animal is most likely twice as wide as tall instead of twice as tall as wide. Could also work up a tab for vehicular size objects such as trucks or armor.

My first hunt with the Chinese pellet gun and cheap 4x scope. 

On my first hunt with the pellet gun, I sat down almost at the top of a hill below some bluffs near the property where my camper is at now. I waited about 20 minutes, being very still. I was mostly camouflaged, except for my blue jeans, which probably looked similar to the large rock I was sitting on anyway. My eye glasses were visible but that was about it.

In front of me I heard nothing, but caught sight of some movement. It was a grey squirrel that moved onto a rock at ground level about 30 to 40 feet from me. As I lifted and moved the gun to get a bead on him, he froze and looked my direction. I thought, “Good squirrel! hold very still while I shoot you.” I put the 4 power scope on him center mass and fired. He then moved very fast while chattering very loudly to a small tree in front of him. Then up that tree. And then did a frantic leap to a larger tree that was behind him. He went around to the back side opposite of me and disappeared . With a semi-auto 22 I would have been able to get in another shot. But with this single shot pellet gun I didn’t have time to reload a pellet before he got behind the big tree.

I went over to the big tree and saw a split at the base large enough for a squirrel to get into. So I guess this was his den tree. I may have hit him or maybe only close. Even if I hit him, it might not have hurt him much if any. I now think I’d had better chance of a kill if I aimed at his head instead of his body. With the pellet gun one must snipe the game.

Zeroing the new BSA scope.

I began to shoot the gun to zero with the new scope. It was shooting all over the place, something was wrong. Sometimes groups would be together and sometimes not. Adjustments to windage or elevation didn’t seem to take effect or even moved the wrong direction. So I inspected the scope mount to see if it was not secure. I found it was secure but the scope rail was not. There was a rivet in the back of the scope rail that was loose. My friend Gary suggested I have someone take a mig or tig rig and tack it. So I called my brother, Mike, who has shops with those rigs. In the photos below, I show where we tacked it. The first tack weld on the rivet did not solve the problem. There was a ring the rivet was attached to that was moving around the housing. We added a tack to the ring and yet there was still a small bit of movement. Last we tacked the rail against the housing and then it was solid.

I began to zero it again and it was quite a ways off in both directions. Like 1.5 feet or so. My friend Gary said we may be able to shim the scope mount rings with cardboard. Its my fault but when we welded it we eyeballed the alignment of the scope rail with the barrel. If we can’t get it zeroed then I may have to take a thin disk wheel and cut the welds and then find a better way to align it, then re-tack weld it.

About Larry Gray

Profile photo of Larry Gray
I'm a middle aged man from the (Arkansas) River Valley area of Arkansas. I grew up on a 200 acre poultry and cattle farm. I have almost 2 years of college towards a computer science degree. My careers so far have been restaurant management 5 years, office work with some computer tech work 6 years and trucking 10 years. I've also had some work in nuclear rad work tool room and part time security. I was in the US Army Reserve for 8 years. And like Jack I've been to Honduras, was there for two weeks once. On the side I have been doing and studying computer work, tech and programming my whole life. I was into caving quit heavily from 1990 to 1993. I explored around 150 caves in northern Arkansas area. I did some rock climbing at the time also, which is part of caving actually. I've been more into fishing than hunting. And my trucking career has given me little time for hunting. I'm currently training new truckers at http://www.willisshawexpress.com Willis Shaw Express the first interstate refrigerated/frozen food haulers. I'm not married yet, never have been and have no kids. I currently camp out on a good friend of mine's land in a 22' camper I own. I have no land yet and I'm currently working to pay off around $25K debt. Some of my prepping is on hold til that is done. Until then I can learn more and prepare for prepping. blog.larrydgray.net

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