MOA Ranging Formula

Discussion in 'Long Range Hunting & Shooting' started by gmkempf, May 6, 2014.

  1. gmkempf

    gmkempf Active Member

    Messages:
    42
    Joined:
    Apr 8, 2013
    Started messing around with this formula using my Vortex Viper HS-T set on 16 power. Target is 36in. tall set at 800yrds. Target is 4 MOA looking through scope. Formula is heigth of target x 95.5 ÷ MOA = yards. The yardage was shot in with my rangefinder. Equation, 36 x 95.5 ÷ 4 = 859.5 tell me what is wrong here. 60 yards is another 10 clicks for me and that is a little over 20in. drop. That my friend equals a clean miss. So is my rangefinder off or or the MOA on the scope or what. Just want a good way to a estimate yardage if the electronics fail. Thanks.
     
  2. 7magcreedmoor

    7magcreedmoor Well-Known Member

    Messages:
    544
    Joined:
    May 23, 2012
    I have not handled the HST, but have both the SFP and FFP HSLRs, and have found the reticle patterns in both to be very close to spec when measured against a test target I created set at 100 yards (laser confirmed). If your rangefinder is on and that 800 yards is a good number, and your mag setting was tight on 16x, then is it possible that your rifle wasn't quite solid enough to get a good visual? 800 yards is pushing it for reticle ranging accuracy even with very high end resolution, and the math indicates that IF the visual measurement had been 4.29 MOA ( kinda hard to discriminate 4 from 4.29 when the hash marks are 2 MOA apart), that would have given you your 800 yards. Parallax can also mess you up as your eye looks away from optical center when trying to bracket the target. The longer the distance, the more the variables start to pile up against you- Long Live the Laser!
     

  3. gmkempf

    gmkempf Active Member

    Messages:
    42
    Joined:
    Apr 8, 2013
    Thank you very much for the reply. So at what yardages do you think the formula can be accurately used to, without to many variables comment I to play.
     
  4. MT4XFore

    MT4XFore Well-Known Member

    Messages:
    708
    Joined:
    Oct 20, 2005
    Is your scope a first or second focal plane? Is 16 the highest power? If it isn't a first focal plane scope, it must be set to it's highest power for the reticle calibrations to be accurate. Hope this helps.
     
  5. brentc

    brentc Well-Known Member

    Messages:
    1,610
    Joined:
    Apr 3, 2009
    Very valid point made here. The HS-T is SFP.
     
  6. 7magcreedmoor

    7magcreedmoor Well-Known Member

    Messages:
    544
    Joined:
    May 23, 2012
    There are a pile of factors that affect the accuracy of reticle ranging. I looked at your reticle on Vortex's website, and one of the limiting factors is the 2moa interval of your grid. The thin lines are 0.15 moa wide, but you don't have anything definitive in between those two measurements. Judging one moa is do-able pretty easily, but for most trajectories that interval is too coarse for ranges 500 yards. At my club we have steel targets of various shapes at each berm, from 200 to 500 and the farthest is 640 (behind the competition target pit). For practice, I make my most careful measurement of each plate, then when I go to paint them I'll get out the tape and see how close I was able to come to the truth. It is a humbling exercise. My HSLR has both 1moa and 0.5moa hashes, which improve my chances. Your tactical reticle is intended for quick ranging of man-size targets, which are taller kill zones than most game animals, hence more forgiving. You said in your OP that the target is 36 inches tall- if there was any angle aspect to your line of sight, this may have skewed your measurement, like when looking up or downhill at a deer makes the body look smaller, leading to an over-estimation of range. When scoping a target of known dimension, you must account for any perspective angle and use the least distorted dimension. And one more note on FFP: the one I use has an open center cross with a 0.15moa dot. A target has to be pretty small to be obscured by that.
     
  7. gmkempf

    gmkempf Active Member

    Messages:
    42
    Joined:
    Apr 8, 2013
    Thank you all for your input. 7mag, you are a professor. Thanks Again, Greg
     
  8. mountainman56

    mountainman56 Well-Known Member

    Messages:
    603
    Joined:
    Mar 27, 2014

    Unless your scope is the new HS-T 6-24x50 in which case the reticle is only accurate at 18 power (I have no idea why they did this. It came as a surprise to me). There is actually a detent at 18 power to help you feel it without looking at the marks.
     
  9. gmkempf

    gmkempf Active Member

    Messages:
    42
    Joined:
    Apr 8, 2013
    Mine is the 4x16 HS-T and all estimates are made on 16x.
     
  10. sscoyote

    sscoyote Well-Known Member

    Messages:
    1,255
    Joined:
    Feb 26, 2003
    Here's what I assume the reticle looks like-- "E". Even the thickness of the lines can throw your estimation/calcs off somewhat. Darrell Holland and I have a theory about interpolating between stadia lines of reticles for rangefinding purposes in that people tend to guess between the edges of the stadia lines and not from the center of the line to the center of the next line. That throws the interpolation off (a lot at that distance) and now the subtension should probably be 4 MOA-.15 or 3.85. Now the equation becomes 36 x 100 / 1.0472/3.85 = 892 yds.
     
  11. sscoyote

    sscoyote Well-Known Member

    Messages:
    1,255
    Joined:
    Feb 26, 2003
    What you may want to do with that reticle is to range a bunch of targets at different distances and then recalculate the actual subtension (the 2 MOA units) from those readings, i.e. say a 10" target at 223 yds. occupies 2.3 2 MOA units, then the equation becomes--

    10 x 100 / X / 2.3 = 223

    X=1.95 inches @ 100 yds.

    ...now do 10 more or so and "reverse-mil" (as above) the subtension, average it and just use that as your "useable" subtension factor in the equation.

    I have found that most reticle-ranging falls apart beyond 500-700 yds. or so depending mostly on size of target relative to subtension unit, magnification, and resolvability.
     
    Last edited: May 11, 2014
  12. sscoyote

    sscoyote Well-Known Member

    Messages:
    1,255
    Joined:
    Feb 26, 2003

    MM, I actually prefer a mil-dot reticle that's not set up for the highest power in a 2FP scope. This way I can apply it at the highest power to achieve a smaller subtension for more accurate windage and rangefinding applications. This is a 6-18X Nikon Buckmaster I set up on a 17 Fireball XP-100. The mil is set up for 12X, but I apply it at 18X where it becomes 12/18x3.6=2.4 IPHY. Great for windage reference and rangefinding. This little rig has had some 1st shots to right around 500 yds. using the lowly 25 Hornady HP in some wind occasionally--

    [​IMG]
     
  13. Canadian Bushman

    Canadian Bushman Well-Known Member

    Messages:
    1,650
    Joined:
    Jan 24, 2012
    I use the formula
    (( target in inches ) \ MOA ) x 100 = distance

    I.E.
    36/4 x 100 = 800

    This is simple enough that most of the time i can do it in my head.
    Often i estimate the largest the target could be, then the smallest it could be and average the two together for my target distance. ( This helps in mirage )

    Then i range the target and see if my estimation was close enough to yeild a hit. I do this before every shot in practice.

    Every so often me and a friend will go do a cold bore contest where we walk to different ranges and leave the rangefinders in the truck.

    6-7 hundred is my max for mil-ing a target.
     
  14. 7magcreedmoor

    7magcreedmoor Well-Known Member

    Messages:
    544
    Joined:
    May 23, 2012
    When using quick math to calculate in your head, beyond a certain range you need to start correcting for the rounding off error- the flatter your load shoots, the farther out you can get by with quick calc, but the round off will get you eventually. After doing the quick math (30in target divided by 6MOA times 100 equals 500 yards) you need to subtract 4.5% to get rid of the rounded off MOA. The 22.5 yard difference can cost you the shot that far out, maybe even closer depending on your load. Or you can use 95.5 instead of 100 if you have a calculator, or are much more confident with your mental math than I can ever be...

    I cheat and lop off 5yds per hundred if I have no calculator. Still a little round off, but can get a bit farther out before it gets me.