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The original range finder, it's right in front of you.

Discussion in 'Rifle Country' started by 1911 guy, Oct 7, 2019.

  1. 1911 guy

    1911 guy Member

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    Allow me to ramble for a bit, it will be worth it if you shoot open sight rifles much.

    We all know that one minute of angle is 1.047" per 100 yards. But most don't know why this is.
    100 yards is 300 feet. 300 X 12 = 3600 inches.
    3600 X pi (because it's a radius around you) = 11,309.73355
    11,309.73355 / 180 (degrees in radius) = 62.83185307 inches per degree
    62.83185307 / 60 (minutes in a degree) = 1.047197551 inches per MOA

    So what does it matter?

    You've got this front sight hanging out there. If you've got a standard AR front sight, it's .072".
    It's also a given distance from your eyeball, depending on your cheek weld. Lets go with 23.5 inches because that's my distance with an A2 rifle.

    23.5 X pi = 73.82742736
    73.82742736 / 180 = .410152374
    .410152374 / 60 = .006835873
    .072 (front sight width) / .006835873 = 10.53267081
    So i know my front sight subtends 10.5 MA

    So:
    Distance from eye to front sight times pi = radius.
    Answer from line one (radius) divided by 180 - one angle.
    answer from line two (angle) divided by 60 minutes = one MOA
    Front sight width divided by one MOA = front sight width in MOA.
     
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  2. MI2600

    MI2600 Member

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    So...
     
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  3. Obturation
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    Obturation Contributing Member

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    If your brain works that way you are a smarter or more complex man than i am. I just aim a freckle high if its way out there. Thanks for the calculation and explanation though.
     
  4. LoonWulf
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    LoonWulf Contributing Member

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    interesting! Id never thought to use a front sight like that.
    you'd only have to do the math once and you would know your subtension....
    would still need to know or guess the width or height of stuff tho.

    I don't use irons much, and carry a lrf, but I'm still gonna play with this idea a bit.
     
  5. Varminterror

    Varminterror Member

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    I use this math often to demonstrate how terribly coarse in aiming precision iron sights really are.

    I would never pretend a front sight could effectively be employed as a rangefinding device - ever. Holding that 10.5moa wide blade on target is one thing, but it’s laughable to think a guy can accurately measure multiples (and fractional multiples) of the blade width on target to precisely range a target is ridiculous.

    Especially considering the illusory zoom of a front sight blade as viewed through an aperture. Varying environmental brightness and varying aperture diameters can make the front blade APPEAR larger or smaller against the target.

    Let’s say I have a round gong at 100 yards. Just about the same apparent width as the front sight blade. Is it 10”, or 12”? Can you tell? So flipping that script - if I have a 30” gong at an unknown distance, again, about the same apparent width as the gong. Is it just a little bit smaller, just as the 10” gong was in the first example, or is it a teeny bit bigger, as the 12” gong was... because the difference in misjudging, misreading the relative size, is 280 yards versus 340. I generally don’t care to misjudge the range to my target by 60 yards. Now let’s say it’s a 4-5” wide prairie dog... it only covers ~1/3 the width of the front sight blade. Is it really 1/3 the width, or 1/4? Or 40%? Is the prairie dog 4”wide? Or 5? Or maybe it’s 4 5/8”.

    The only conditions in which such crude, inaccurate, imprecise systems can work are when there’s a massive margin for error, effectively, within the maximum point blank range of the cartridge, aka, where rangefinding isn’t pertinent.
     
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  6. Tommygunn

    Tommygunn Member

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    I'm sure this guy could do all that math in his head to the 100th decimal place in one tenth of a second.:p
    I have a bit of trouble with it, though. :thumbdown:
    :D
     

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  7. 1911 guy

    1911 guy Member

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    Using iron sights as coarse rangefinding devices has been taught for a very long time. It's only fallen out of use with the advent of range finders and the ubiquity of magnified optics now.
    The math only needs to be done once. The most common usage is torso sized targets at unknown ranges. A given number of sight widths means approximately X range and so clicks or holdover are estimated within a fair degree of accuracy.
    No, it's not a replacement for a varmint rifle and laser range finder on tiny targets. But I know a couple people who've gotten rounds on target with irons farther than many people can hit with optics.
     
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  8. Fine Figure of a Man

    Fine Figure of a Man Member

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    Seems like a lot of arithmetic to go through for a guesstimate.
     
  9. HB

    HB Member

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    I have used a similar method for in range/out of range.

    I was forced to use a shotgun with sights for deer hunting one year. The front sight was the same size as a paper plate at 60 yards. Thus if the blade was smaller than the deer vitals, it was in range.

    This was crude but effective. Rather than determining precise holdover, it is useful for a known target size in unfamiliar terrain.

    It is helpful when youre about to take a snap shot and final verification is the bead inside the deer.
     
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  10. Chuck R.

    Chuck R. Member

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    But, it's an 'educated' estimate!

    A couple of us just ran a similar exercise a few weeks ago, but by using Leupold Duplex reticles, which believe it or not were initially designed to allow "subtense" range estimation on a deer sized target. On average we came out within + or - 20 or so yards on a tgt out to 400 yards. Not laser accurate, but better than even an 'educated' guess.
     
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  11. pblanc

    pblanc Member

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    Alright, I know this is going to sound pedantic, but this is a pet peeve. I cringe when I hear someone say "one minute of angle is 1.047" at 100 yards".

    Minutes of angle are units of angular measurement. A minute of angle is 1/60 of one degree of angular measurement.

    An inch is a unit of linear measurement. A unit of angular measurement cannot become a unit of linear measurement and vice verse.

    One minute of angle subtends one minute of arc on the circumference of a circle the center of which is at the vertex of that angle of minute. If the radius of that circle is 100 yards, the length of the arc subtended is 1.047". Note that this is a measurement of arc length, not a straight vertical distance as we are usually measuring off a target. But because the unit of angular measurement is so small, the curvature of the arc can be disregarded.
     
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  12. BigSteve57

    BigSteve57 Member

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    I've used duplex reticles to guesstimate range for a long time but never thought to use irons for that.
    I've even had to find scope sub-tensions myself using a sighting in target.

    I just figured out that my 1 in. wide thumb held out with my arm fully extended 24.5 in. from my face subtends an angle that results in an elevation of 146.938 in. at 100 yards. Not too useful for hunting I guess. :D
     
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  13. Dave DeLaurant

    Dave DeLaurant Member

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    Boy, I wish my eyes could still resolve both a rifle front sight and a distant target clearly enough to experiment with this!
     
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  14. Bwana John

    Bwana John Member

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    How about using 2 arctan ((1\2 group size) /distance to target) to use linear measurements to determine an arc measurment?
     
  15. pblanc

    pblanc Member

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    I tend to agree. At best you will get only a very rough estimation of range, and then perhaps only if you have a very steady hold and excellent eyes. Lighting conditions can also dramatically affect apparent target size and front sight blade/post width.

    I was at a recent event in which this was well-demonstrated. IDPA targets of known width were placed at unknown distances from the firing line from about 150 yards out to 300+ yards. There were around 9 shooters. One had A2 style iron sights. The rest had optics of various types, ACOG, LVPOs, rifle scopes with various types of reticles including mil-dot. Some of the targets were in full sun, some in shade. We were asked to sight in on the targets and were allowed to take as much time as we liked, using bags or bipods for support to steady our hold on the targets. Shooters could also use calculators to do the necessary math if they wished.

    With all of these crutches, I was pretty close on about 4 of the targets, and had the closest range estimate on one using my scope reticle (a Nikon P-Tactical 223 4-12x40 BDC), but on one target in the shade, nobody was even close. The closest estimate was around 75 yards off.

    I have no confidence in being able to make a reasonable estimate of target range quickly, using iron sights or a scope reticle, especially if I had to do so shooting offhand without rifle support.
     
  16. Varminterror

    Varminterror Member

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    I made this graphic a few years ago for a rifle course I occasionally have offered as part of the differentiation between IPHY (aka “shooter’s MOA), true MOA, and then further explicate the difference between subtended arc length and subtended chord length.

    We’re getting pretty deep out of sigfigs before a 100yrd radius, measured typically at best to the nearest 1/2yrd, shows any difference in arc length vs. chord length matters - especially when considered in relation to our aggregate group sizes.

    0501F081-A3F7-4230-BE3D-2DAC627C24D0.jpeg

    I have another similar graphic which I created (but apparently don’t have on my phone or in my Flickr...) a few years ago which reflects 100 yards as the perpendicular distance to the target, resulting the hypotenuse is slightly longer than 100.000000 yards (~100.00000106yrds). The chord length then becomes 1.047197558”, which is intuitive, since the target is slightly farther away than the exaggerated 1moa pie wedge pictured above. The problem here, the graphic is harder to understand - as it reflects the 1moa group dispersion as the cross section of a projected cone made by a 1/2moa right triangle (two triangles instead of 1). Technically, this would be more correct for the system, since we don’t necessarily measure the hypotenuse, we measure the normal (perpendicular) distance... again, so deep into insignificant digits and so far outside of the precision of our tools, it’s irrelevant.

    I’d agree, calling out the colloquial grammar of saying “1moa is 1.047 inches at 100 yards,” is nothing but pedantic. Angular subtension is implied by offering “rise and run.” I’m rarely accused of brevity or concision in how I communicate, but in teaching MOA to hundreds of students for a couple decades, and I do try to police myself to say “1moa subtended approximately 1.047” at 100 yards,” but in conclusion or synopsis, I can be assured I say “is” instead of “subtends.” Range and dispersion, “rise and run” are given by the statement, with angular subtension implied. I’ve played with different analogies over the years with different audiences - driving up a hill, projecting a cone, etc - many analogies can effectively communicate it to the students. I have found that many students honestly cannot comprehend a system of angular measure, cannot comprehend “angles,” and my job in my shooting classes isn’t as a middle school geometry teacher, in those classes, I’m a shooting instructor, so I teach marksmanship skills and tools. Some students will be skilled enough to understand how the sausage is made and will be better marksmen for it, some won’t get it, but can still be successful marksmen because the skill doesn’t rely upon a high level understanding of geometric principles. For the average shooter - IF they can comprehend angular systems, subtension is implied by giving rise and run. Folks who really get it will understand the difference between a 100 yard hypotenuse and 100 yard leg, folks who kinda get it will understand subtension is implied because rise and run are given, and those who don’t comprehend angular measure but can use a calculator won’t ever get it unless they go back to middle school geometry, but they can still deliver impacts at 1,000yrds because they only need to know the formulas, not the underlying science. Folks who can’t operate a 4 function calculator do exist, and it’s very challenging when one shows up for a rifle marksmanship course - but in general, they can be coached well enough to limp through and make impacts at range... Engineers, architects, superintendents, and hammer jockeys... Some can, some know how, some know why...
     
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  17. 22250Rem

    22250Rem Member

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    ....... I've heard of that before... a long time ago.... from old timers who never used scopes. Also reminded me of the writings of Elmer Keith concerning some of his long range prowess with a .44 cal. iron sighted revolver. After estimating distance he instinctively knew how high to hold that front sight after all those years of doing just that and almost exclusively with the same load. He had it down to a science; and he wasn't just bragging at the keyboard. He was witnessed by reliable people countless times. In his book, "Sixguns", "1961 Edition" he even devotes an entire chapter to it. That stuff isn't heard about very much nowadays but there is validity to it and it was once a regular thing. Interesting to see this topic come up again. Haven't heard about it in a long time.
     
  18. Varminterror

    Varminterror Member

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    Here’s a similar hopeless gimmick which has been used for a long, long time...

    Guys used to use this kind of “rangefinder” much in the same way as this front sight method. You first “calibrate” the string; get 100yrds, measured, from a reference target of a common height - say, a 6ft man - set the slider at 100, and then move the viewer closer or farther from your face until it tightly fits the reference in the gap. With the string in your teeth, held taught, tie a knot to allow the same distance to be replicated.

    From then on, you hold the knot in your teeth, slide the bar to tightly bracket a reference sized object - say, another 6ft man - and the reading gives you your range.

    Same issues as using your front sight - the precision isn’t there. What if the guy is only 5’9”? What if you can’t accurately bracket the reference? If the slider is between two marks, how closely can you interpolate?

    For any range that these crude methods work to within even moderate accuracy, you simply don’t need a rangefinder because you’re not far enough away. These gimmicks are distractions which lead unwitting newbies away from techniques and gear which can actually help them in the field.

    829E3F08-A6E5-4139-B986-2A17303AA2E8.jpeg
     
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  19. roscoe

    roscoe Member

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    It can in a non-Euclidean shape space. Not to be pedantic . . .
     
  20. Old Stumpy

    Old Stumpy Member

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    Actually, he was puffing on a stogie, drinking Jack Daniels, and 2 finger pecking at his typewriter, telling it true.
     
  21. Old Stumpy

    Old Stumpy Member

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  22. MTMilitiaman

    MTMilitiaman Member

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    Knowing how large your front sight is in relation to the target is useful knowledge. This stuff used to be considered basic "shooting a rifle" stuff. Now we have a couple generations of shooters brought up on magnified optics, and fundamental marksmanship skills are going the way of the VCR. That doesn't make it any less relevant or useful.

    Unless you're using a laser rangefinder, your range estimation is just that, an estimation. You use mil or MOA based rangefinding scopes in the same way. They require an estimate on the size of your target and some math. You can do the math ahead of time and have it available on a card, but you still have only a best estimate of the size of your target. The magnification can help with you get more precise measurements, but in the end, you still have to practice to use these systems effectively.

    I qualified at 500 yards with an A2 in boot camp. I was then issued an A4 with an RCO. The RCO had a rangefinder and bullet drop compensator in it, and was amazingly intuitive and simple. It made people who could barely qualify with irons scary at 600 yards with a little practice. But it still required you to estimate range based on 19 inch brackets. If your target wasn't 19 inches, your range estimation would be off. You were expected to practice with it enough to be able to tell how large your target was and get an accurate range with your optic. I have a TA11J on my AR10, and it is the same deal. The 3.5x magnification helps some, but you still need to practice, and having that size reference is still useful.

    So if you know your front sight is 10 MOA, and that your typical across the shoulders fighting age male is say, 20 inches, then with practice, you can judge range fairly effectively. If your front site goes shoulder to shoulder, that guy is probably about 200 yards. If target only takes half the distance of your front site, he's probably closer to 400 yards. Is this perfect? No. Is there environmental conditions that can affect it? Yes. Does it take practice to use effectively? Yes. But is having this information and even a rough estimate of range useful? Absolutely. Having any tool in the tool box that helps you get the first hits on target it useful.

    I don't see why people are so quick to poo-poo useful information. Maybe it is because I still practice with iron sights.
     
  23. Old Stumpy

    Old Stumpy Member

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    Agreed. The same thing can be applied to someone hunting with a rifle with buckhorn sights. If you know the sight picture in advance in relation to a particular game animal at 100 yards, and also at 150 yards, then you can decide where to aim, elevation-wise.
    You could practice this with a full size game animal paper target at different distances at the range. You would then know the bullet drop (if significant) at these different ranges and where to place the sights to assure a kill zone hit.
     
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