Outdoors, Green Living, Homesteading, Sustainable living, Green Building

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

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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 at 180 and 2PI at 360 degrees. Milli-Radian is 1/1000th of a Radian. Or it means that the image in the scope at 10x power is 1/1000th the actually size of the object. So a 1 meter tall target at 1000 meters is 1 mil dot tall. A 1 yard tall target at 1000 yards is 1 mil dot tall. A 1 foot tall target at 1000 feet is 1 mil dot tall. A 1″ target at 1000″ is 1 mil dot tall. A 1 cm target at 1000 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 its slightly less than 1/2 mil dot tall. A Mil dot at 100 yards is 3.6″ (1/10th of 1yd(36″) at 1000yrds) There is nearly 17 Mil dots in 1 degree and 1 degree is 60 MOA. Or there is .28 or close to 1/3rd mil dots in one MOA. And one MOA at 100yrds is close to 1″. 200yrds 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*1000). % 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 yrds. My new BSA scope has 1/4 MOA adjustment for both windage and elevation. This means 1/4″ at 100 yards. So this would be 1/8″ at 50yrds or 1/16″ at 25yds.  I have decided to zero my Chinese air gun at 20yrds because it is shooting at only 420 fps. This gives me a kill zone from 4yds(12ft) to 22yds(66ft) of 2″ diameter.

Once zero is set it can be moved with the elevation adjustment. At 20yds 1″ is about 20 clicks. A person could also re-zero at 25yds, 30yrds and 15yrds 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.

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 100yrds from left to right or up to down. Many of these scopes are 30″x30″ at 100yds. So with lack of proper information I may have to use empirical methods for figuring this out with my particular scope. Once I have done that I will publish the results here. This process should be simple enough. I will setup something with 1″ or 2″ marks on it at 100yds from viewing position. Then count the number of marks from center to left or whatever. Double the results.

Saying the inner cross hair boxes 30″x30″ at 100yrds 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 25yrds then it would fill the box top to bottom. Its a matter of ratios. Its 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 7.5yds to 100yds to infinity. It has markings on it for ranges between 7.5 and 100yds. As that article suggest 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 to use this adjustment in ranging targets.

Basically 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 cross hairs. 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.


2 responses

  1. Pingback: .177 Air Rifle AKA BB/Pellet Gun | Larry D Gray

  2. Steve Hugel

    Larry, good stuff here. I have also put a lot of research into reticle-rangefinding and have found the 2 most important and useful concepts are–
    1) The “mil-ranging formula” (the algebraic simplification of your angular ranging reference above) defines rangefinding with any multi-stadia reticle subtension from mil-dot to Balistic Plex to simple plex to even archery sight pins if one wanted to, and-
    2) Reticle subtension is ~inversely proportional to magnification in second focal plane reticles.
    Check out my Youtubes (sscoyote) for details of the abovementioned systems.

    July 4, 2015 at 2:03 am

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