Range estimation with a duplex?


Jun 14, 2003
Red Sea
I know it's POSSIBLE, but anybody here ever range-estimate with a duplex reticle? What kind of accuracy can you get after you practice for a while?
I know mil-dot is definately the way to go for maximum accuracy but I'm interested in seeing how good a duplex can be.
The duplex reticle can be used for distance estimation to good advantage if the user makes an effort to learn some basics.
I suggest placing an 18" square (plywood, cardboard or whatever) out on a rock, post or easily observed location. Then back off in 100 yard increments and note (write down) what power your scope must be adjusted to, so that the 18" is bracketed by the posts. Way out there use the crosshair and a post. You can get some good data that you can use in the field, at X power if the buck's chest is bracketed then he is about Y yards.

Many reticles are already in 18" separations between the posts (some are 16") so that makes it easier and for some you simply read a second set of numbers on the power ring for the rough distance. This is pretty handy, but a good laser beats it hollow. How accurate, I have never tried to tie that down, I'm guessing 20 yards or less but suspect greater distances will effect this. There used to be some scopes with numbers inside that if you bracket the target a number would be visible in the field of view that approximated the distance. Even used in some sniper scopes (Redfields for instance).

In my experience Mildots are field useable for hunting if you know some dimensions of your critter and have a cheat-chart made up to give you quick readings (some guys can memorize the necessary numbers). Sometimes there is time for Mildot Masters and even a calculator but, bottom line is that stuff can be eliminated by a laser.
Hey Ian, any more updates on the Nikon tactical? I noticed that only the 2.5x10 has the illuminated reticle or am I wrong?
You are correct, only the 2.5-10 has the illumination. Nikon uses a neat system, you can choose between red or green with several brightness levels for each.
The Nikons are working great, the original prototypes have several thousand rounds through them now and no problems.
I took a new 2.5-10 Nikon and a 3.5-10 Leupold LR M1 on a pdog shoot, put them on identical rifles and everyone who shot them preferred the Nikon, unanimous. The Leupold is a fine scope and I really like it, but the Nikon is brighter and better definition to my eyes. Turrets have been excellent, you can say that about both scopes.
How is the weather down in your beautiful city, hot I bet. We are in the mid to upper eighties but usually a nice breeze that helps a bit. Imagine "Santone" is pretty muggy.
Funny you should ask the question about rangefinding with the duplex reticle. I've got an article going into Precision Shooting publications about a guy i met that has manipulated the range estimating system (RES)developed for several of the Vari-X III scopes (Pentax + Burris also offer a similar system as well) for any spp. of game he may be hunting. Ian M explained the system pretty good. This friend of mine has not only used the sytem on game but convinced a friend of his to use it in some of the sniper competitions as well, and since applying the system he has won the Steel Safari in New Mexico twice now.

Leupold's RES is based on a deer sized target measurement, back-brisket, of 16", such that the center-top of post gap of the reticle subtends this measurement @ 100 yds. on the lowest magnification (I believe Burris + Pentax use 18" as a standard). If the deer is smaller than the gap then simply increase mag. until it fits, and voila read the back of the ocular for range. Now if you're shooting at something smaller or larger, a simple recalculation of the power ring is all that's necessary. But as my friend found out, trying to change magnification on the power ring in the field is difficult at best, so what he did was simplify the entire concept by doing the following-- he set the scopes mag. at the maximum point blank range that he could hit a coyote with the load he was using ( let's just assume it's 300 yds. for a coyote-sized target). That would mean that the center-post gap would have to be adjusted to subtend 11" (for a coyote) @ 300 yds. Of course, then all the shooter needs to do is simply bracket the coyote, and if it fits or is bigger then aim center mass and shoot. If the coyote was 25% smaller then he must be 25% farther away, etc. Using this system he has made 1st shot connections on all manner of game animals farther than he ever did before. The neat thing about all this is that the relationship between image size, magnification, and reticle subtension on a variable-powered scope is linear, making calculations fairly simple to perform. Since i've been messing with this, i've probably learned more about my scope than i ever new before, and if you take the time to investigate it you'll find that there are a lot of ways to manipulate the scope to work for your particular needs. It really is fascinating stuff, and if you have Exbal ballistic calculator it opens up even more fascinating possibilities, as he has an option to optimize factory, custom, and even plex reticles for your particular load. Fun stuff.

[ 07-19-2003: Message edited by: sscoyote ]
Thanks for getting back to me Ian, thought I was going crazy trying to find the 4-16 with the illuminated sight. Whats the heaviest recoiling thing you have fired under it?

Mid 80's huh
. Well after Claudett decided she didn't want to visit Corpus Christi, she gave us waves of thunderstroms, high winds (I've never seen my trees bend over to touch the ground) and low 90's with 100 percent humidity as she hit Victoria.

Cancelled plans to go check out the blinds until the fields dry up maybe 2 weeks
Now that would have been some windage! We have twisters up here, even had one recently up in the northern forest. Killed the hell out of a lot of trees, not many people up there so no-one was hurt.
Have shot the NIKONS on .300 Win mag., nothing bigger to date. I am very confident that they will take some abuse, tho.

What your friend is doing is slick and has to work. I made up a simple chart that I make on a label for the Duplex ranging, just happens to be near identical to what you guys are doing.
A lot of what you outlined will also work with mildots. I believe that they are more useful as you have multiple "contant" reference points that can be varied by moving the power ring. Duplex pretty much only gives a person one holdoff, on top of the post as a constant. Mildots enable a similar quick-read and also some very nice holdoffs.

Any preference, pointed posts vs flat-tops?
You know Ian, i've never worked with a flat-top post, as seems most of the more common plex posts are pointed. I've only used this method myself once with my 700 6AI. Had a 4-12X Burris Mini on top. Exbal calculated the plex zero, and i had a Stoney Point target turret screwed on. I can't remember offhand what the post was zeroed for, but i ranged a little cactus on a teepee butte at 600 some (been awhile-- can't remember exactly). Ran the clicks in, got steady and shot, and darn if i didn't nail the cactus. (even saw the hole in it). I was coyote hunting at the time-- sure wish it was one of them-- oh well, next time. I know this has nothing to do with ranging with the reticle, but it was fun nevertheless. This guy I was speaking of has got quite a shooting savvy, and has made the system work on several elk and antelope, as well as coyotes. I've been working with the Burris Ballistic Plex reticles myself recently, and that's been a bunch of fun, especially in combination with Exbal.
I've ben using duplex reticles for range estimation for many years. I do it a little differently than the other posts on this thread; for many years I have kept a record of the back-brisket heights of animals. Based on these heights at 100 yds, I have a yardstick set up with these heights divided by 1/4, and set up the yardstick in my back yard at 25 yds (as opposed to 100). When calibrating a scope for AZ whitetail, for instance, which average about 14" for a good buck, I calibrate the scope to the 3 1/2" mark (14" divided by 1/4) by turning the power adjustment ring until the duplex steps just cover. I then record the power.

Obviously, if the deer on the hoof extends from step to step it's 100 yards away; if it extends from a step to the crosshairs it is 200 yard away, etc. Using this method, I can pretty accurately range to about 500 yards; then it gets a little shakey. Not as good as a rangefinder for the long shots, but faster. Also, not real good for a deer other than broadside.

I have 2 Leupold 2 1/2-8's and the duplex extentions are quite a bit different! The first was about right for elk, covering 24" at 2.6 power, but the second one couldn't be turned down enough to cover the 24". Also, don't assume that if a scope covers 12" at 6 power that it will cover 24" at 3 power; they do not have straight-line advances!

This system has never failed me, but I failed it once because I just didn't believe the scope. In the reticle the deer appeared to be in excess of 500 yards and I refused to believe that it was over 400. I held a little over its back and hit it in the knee! I eventually got it, but it took some chasing. So much for the 8 deer with 8 shots record of my 25-06.
I have done this for years. Animals that live in areas with little background cover for a good laser lock like antelope and caribou render laser rangefinders useless. I set the average of my mid range trajectory the same as the back to brisket measurement of my quarry. For instance caribou average about 21". When I zero my rifle at 650 yards the mid range trajectory is within 3.5" up or down of 21" from 175 to 575 yards. Simply adjust the power ring to frame the caribou between the top duplex and the crosshare which measures 21" for you. Now the point of your top duplex is 21" high at the point where the caribou is standing. Hold that point dead center of the chest and shoot. If the caribou is between 175 and 575 you will hit not more than 3.5" high or low, or in other words very dead caribou. If he is further than 575 then you have two other sights to use which are the crosshare and bottom duplex. I know exactly where these hit also. In hunting situations many times you don't have time to turn clicks and this method is very fast. I have been doing this for thirty years and everyone that hunts with me are amazed how I never miss at long range. This method works very well out to the limit of where I shoot at game which is about 850 yards. Usually I can figure a stalk to get within this range. Just keep in mind things that can decieve the system like steep up or down shots.
Welcome Oldtimer. You say this system has been working for you to 800+ yds.?? Taht's excellent. I would have thought that resolving power of the scope as well as environmental conditions would put a shorter limit on this "optical" rangefinding system. But if you say you're going out that far then now you've got me thinking. Sometimes i get in trouble when i do that.
You mentioned that you have some measurements of big game, could you share them so we can add to our accumulation of measurements. More info the better.
Had a question about the use of plex reticle for second zero reference from Vic in MT (sorry Vic had a computer or maybe computer user glitch and lost it). Here is what i've come up with from my research. I've created a plex reticle worksheet that can be used to calculate a rangefinding system and additional zero's for any plex reticle. At exactly 102 yds, from the kitchen table out my front window is a sign post with holes along it that measure exactly 1" apart. This provides a quick and simple measurement of the reticle subtension. When a friend comes over we set his gun up on bags on the kitchen table ( when the wife's not home) to measure the reticle (at the scopes highest mag.). As soon as i obtain the mearurement i write it on a sticker and put it on the inside of the turret cover for future reference if needed. If he knows the m.v. of his load i then follow the following calculations to maximize the potential of the reticle.
1) Run the loads ballistics figures through Exbal (or any other ballistics program) to determine the maximum point blank range (MPBR) of the intended target.
2) Then calculate the mag. needed for MPBR for rangefinding purposes using the following LINEAR formula: highest scope mag./ target measurement= X mag./reticle subtension @ highest scope mag. @ MPBR. Now this means that when you adjust the scope to this calculated (X) mag. when the target subtends the reticle or is bigger, just aim dead center and shoot for a certain (theoretically) hit. Now here's the nice little additional feature that one may wish to adopt. Suppose the rangefinding mag. needed on a 4-12X scope is 5.5X. well i don't know about you guys but when i'm presented with the opportunity (time actually) to use this system, i want the highest mag. i can get for a calculated shot-- right? So instead of using the center x-hair- post gap for rangefinding try using 2X that mearurement (11X in this case) for the post-post gap instead. This should provide a higher mag. for rangefinding purposes, as well as additional reference (the center x-hair) for approximating range at longer distances.
3) Now in order to calculate the new 100 yd. subtensions for bottom post zero @ MPBR mag. apply the following formula: X" @ 100 yds. @ MPBR mag./highest scope mag.= subtension @ highest scope mag. @ 100 yds./scope mag. @ MPBR. From here i use Exbal to calculate the new zero for the lower post (at the mag. needed for MPBR rangefinding). I can also punch in 2X that measurement for an aditional zero (in this case suppose that zero range is 625 yds. If i've calculated this distance to the target simply hold the lower post on the target, note where the x-hair is then bring the lower post to that spot and shoot--- theoretically, mind you). trying to create a mathematical model for this calculation is no longer linear and requires algebraic formulas since the bullet trajectory is parabolic in nature.

Of course, this is all theoretical since i just thunk it up recently, but it should work (check my calculations please administrators) since the relationship between reticle subtension, apparent image size and scope magnification is linear, and should at least get you on paper, and while you're testing this mathematical model it might be a good idea to check to make sure the MPBR for rangefinding is right also. The nice thing about this system is that it can save time and wasted bullets at the range.

Sounds like Old Timer's got the system down to a science as well-- even more since he appears to be using his upper post as the original zero for MPBR.

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Ian, Bob Hagel's book, "Game Loads + PracticalBallistics for the American Hunter" has published measurements for buck antelope and bull elk(Rocky Mtn. i assume). Bull= 32-34" back-brisket (b-b), vital area= 22"; buck= 15-16" b-b, vital=10". Obviously approximate figures.
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