bracketing game with scope

Discussion in 'Long Range Hunting & Shooting' started by new shooter, Oct 15, 2009.

  1. new shooter

    new shooter Well-Known Member

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    I have put 400 rounds through my 338ax. I am shooting from 800 to 1200 yards real will using my swarovski rangfinder. What i would like to know is there a way to get the distence to the elk by using the the scope cross hairs i have a night force 5.5x22-56.It would be great if there was a way to bracket a deer or a elk if the rangefinder didn't work. Also what are the cross hair above the center used for? Thanks
     
  2. MontanaRifleman

    MontanaRifleman Well-Known Member

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    A typical mature bull elk will be 30" from brisket to top of back. A mulie Buck will about 18", and an antelope 16". It's basically a matter of doing the math for what ever reticle you have. I would say it's fairly reliable out t0 400-600 yds. IMO, 600 would be pushing it. I used that method to range an antelope buck @ 300 yds. Seemed to work very well to there.

    You could make yourself a siloutte and pick some random distances to see how well do.

    -Mark
     

  3. sscoyote

    sscoyote Well-Known Member

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    Yes u can use the stadia in a reticle to measure the distance to a tgt. of KNOWN dimension. The best way to accomplish this is to apply the "modified mil-ranging formula." All rangefinding with reticles (stadiametric rangefinding) is based on this simplified equation. Here it is (inches to yds.)--

    tgt. size (") x range of reticle subtension (usually 100 yds.) / reticle subtension (") / quantity of gap tgt. occupies between stadia (decimal equivalent) = range (yds.)

    ...looks complicated, but quite simple to apply--

    Example--sounds like u have the NP-R1 reticle (maybe R2?). If it's the R1 then i think the stadia to stadia gap is 1 inch per 100 yds. in that reticle (better check that 1--don't know for sure). Suppose u look at your big bull elk at an unknown distance and u see that he occupies 3 and 1/4 of the 1 IPHY stadia units. Most folks say that the avg. bull elk is 25" back to brisket, but this guys a biggie so lets give him 2 more inches at 27. Now just fill in the variables in the equation--

    27 x 100 / 1.0 / 3.25 = 830 yds.

    ...BUT...it's not really that easy oftentimes. Obviously u need to guess the tgt. size correctly, and guess the "gap" correctly too.

    Now let's see how far off u will be if u're off in tgt. size by only 1"--

    2600/3.25=800
    2800/3.25=861

    so as u can see +/-1" gives 60 yds. variation in range estimation, and thats assuming u've guessed the gap accurately to a level of 1/20th of it's subtension (.05).

    It should be obvious that as range increases the error increases geometrically.

    I've found reticle rangefinding with scopes is fairly accurate to 400-500 yds. and then drops off dramatically beyond that.
     
    Last edited: Oct 16, 2009
  4. sscoyote

    sscoyote Well-Known Member

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    Looks like MR got to the response key before me, so maybe the 30" is more right than my guess at 27. That's probably more correct than mine, but at least my math's right.
     
  5. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I have a bull elk on my wall that measures 30" and I confirmed the measurement with a taxidermist who said that is the measurent for most mature bulls. You'll probably get a little variation between individuals, but 30" is the norm.
     
    Last edited: Oct 16, 2009
  6. liltank

    liltank Well-Known Member

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    so hows that math work out for a regular old mil dot?

    Tank
     
  7. sscoyote

    sscoyote Well-Known Member

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    See where the "reticle subtension (")" is? That variable would then become 3.6", and if somebody had a mil-dot reticle and looked at the same tgt. at 830 yds. right next to this guy who's using the Nightforce NP-R1, here's what it would "gap" in his reticle--

    27x100/3.6/x=830

    x=0.9 mil.

    Now if i were looking at that tgt. with my Nikon Buckmasters mil-dot at 18x (now 2.4 inch per hundred yds. between dots) instead of the mil-cald. power of 12 here's what it would gap in that reticle--

    27x100/2.4/x=830, x=1.35 mils--
    [​IMG]

    ONE of the nice things about understanding how to manipulate this formula is that if u have a mil-dot reticle that's cald. for a power that's lower than the highest (mine above), u can "mil" at a higher magnification which allows u to define the edges of the tgt. better--most of the time.
     
    Last edited: Oct 16, 2009
  8. sscoyote

    sscoyote Well-Known Member

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    Oh well--looks like i erred once again-- the NP-R1 is actually true MOA according to the catalog. That means it's 1.0472 IPHY between lines. So u can substitute that figure (subtension) into the equation.

    Now here's something that Darrell Holland's teaching his students in his classes that i bet not many r doing to have the most amount of accuracy possible from rangefinding with the reticle. This is actually something i stumbled across myself a few years ago while using the Burris Ball. Plex to range with. When using line stadia for rangefinding the shooter should subtract 1 line thickness for calculating his subtension factor. The reason is that people tend to gap BETWEEN stadia lines not from center to center. It may seem like a moot point, but it could make a difference. I mean u have to use a factor anyways--might as well be as accurate as possible, right?

    That means that his subtension factor should be 1.0472" - .065" (thickness of 1 NP-R1 stadia line) = .9822"

    Now substituting that into the original equation we now get--

    2700/.9822/3.25=846 yds., a difference of 16 yds. (could make a difference--better have a steady rest to range with that degree of accuracy with that reticle at that range at that power).


    ...now if a shooter had the 12-42 NXS and had the NP-R1 reticle he'd have a unit of subtension of 0.51 IPHY @ 42x which would be about as good as it gets for reticle-rangefinding with any scope maker out there.
     
    Last edited: Oct 16, 2009