Crosshair positions for Long Range Shooting

Discussion in 'Long Range Scopes and Other Optics' started by Loggerhead, Jul 12, 2006.

  1. Loggerhead

    Loggerhead Member

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    I have a B&L 5x15x50 scope without target knobs. No one seems to have them including Kentonidustries that will fit my scope. I was wondering if anyone using a 300 yard zero with a .300 Win. Mag, could tell me what yardage the tip of the bottom post of the crosshair would be with a 165 gr Sierra @ 3100 fps.
     
  2. Dave King

    Dave King Well-Known Member

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    You'll need to measure the value between the crosshair "+" center and the tip of the lower/bottom post for starters.

    Bag the rifle or scope up and measure against a measured grid square at 100 yards, you might get by with using a good yardstick. Once you know this crosshair to lower post tip linear distance at 100 yards (100 yards measured, not guessed) you can calculate the MOA value easily. (You can measure this at 100ft, 75 feet or any known distance so long as you are careful about the distance and getting a good measurement against the reticle.)
     

  3. Loggerhead

    Loggerhead Member

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    Ok. does the scope have to be at a certain magnification and how do I calculate this info once I get it.

    Thanks
     
  4. Buffalobob

    Buffalobob Writers Guild

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    Screw off the turret cap and carefully measure the inside diameter of the cap where the threads are with an accurate caliper. Then post that measurement here on this thread or else email it to me. I have a very very old target turret that is very small in diameter and I cannot remember what scope it went on but it doesnot fit anything I currently have.

    If your inside measurement is close to the inside diameter of this turret then I will mail it to you to try. If it doesn't work then too bad. It is of no use to me and is just sitting in an old tobacco tin.

    Let me know the inside diameter (or you can measure the turret threads on the scope).
     
  5. Dave King

    Dave King Well-Known Member

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    Measure the distance with the scope at high power and them measure again at low power, it the measured distance doesn't change then the power setting won't matter, otherwise you'll need to calibrate the scope at a set power value.

    If you measure at 100 yards and the value is for example '8' inches from center "+" to tip of the bottom bar you can (approximate value here for ease) straight convert to 8 MOA (in reality it'd be something like 7.640879 MOA but we'll just call it 8 because I'm sort of lazy).

    If you zero your rifle to be "dead on" at 300 yards you then determine the trajectory further out where you need 8 moa of correction UP and that is your distance where the tip of the lower bar is the zero.

    <font class="small">Code:</font><hr /><pre>

    Range Vely Energy Total Bullet MOA
    yds fps ft/lbs Drop Path Drop
    0 3050 3924 0.0 -1.8 0.0
    25 3006 3811 -0.1 -0.1 -0.5
    50 2962 3701 -0.5 1.2 2.3
    75 2920 3596 -1.1 2.4 3.0
    100 2877 3491 -1.9 3.2 3.1
    125 2835 3390 -3.1 3.8 2.9
    150 2794 3293 -4.4 4.2 2.7
    175 2752 3194 -6.2 4.2 2.3
    200 2712 3102 -8.1 4.0 1.9
    225 2671 3009 -10.4 3.4 1.5
    250 2631 2920 -12.9 2.6 1.0
    275 2591 2831 -15.8 1.4 0.5
    300 2552 2747 -19.0 0.0 0.0
    325 2513 2664 -22.5 -1.8 -0.5
    350 2474 2581 -26.4 -3.9 -1.1
    375 2436 2503 -30.5 -6.4 -1.6
    400 2398 2425 -35.1 -9.2 -2.2
    425 2360 2349 -40.1 -12.5 -2.8
    450 2323 2276 -45.4 -16.1 -3.4
    475 2286 2204 -51.2 -20.1 -4.0
    500 2249 2133 -57.5 -24.6 -4.7
    525 2213 2065 -64.0 -29.5 -5.4
    550 2177 1999 -71.1 -34.8 -6.1
    575 2141 1933 -78.7 -40.7 -6.8
    600 2105 1869 -86.9 -47.1 -7.5
    625 2070 1807 -95.3 -53.8 -8.2
    650 2036 1748 -104.0 -60.8 -8.9
    675 2001 1689 -113.5 -68.6 -9.7

    </pre><hr />


    Notice in the above chart (3050 fps muzzle, 190gr bullet .550BC, 59degrees, 500' above sea level) that at 300 yards the bullet path and MOA drop are -0-. Notice that when the MOA drop = ~8 the distance (yards) is between 600 and 625. This yardage of 600 - 625 is where you should be Point-Of-Aim = Point Of Impact when using the tip of the bottom bar --- this is an example --- you must measure your scope and recalculate.

    (Okay guys, did I get it close to correct? Someone double check me but I should be fairly close using the ~MOA values)
     
  6. Loggerhead

    Loggerhead Member

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    Buffalobob, I emailed you the measurement.
    Dave, thanks for the info. The chart looks similar to others I have run but didn't know how to use them.
     
  7. Loggerhead

    Loggerhead Member

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    Ok Gents,

    I set a scale of 1" marks at 50 yards and at 10X had a total of 3" of coverage. I would think that would be 6" @ 100 yards. Is that correct? Therefore according to my ballistic program, by setting the mag to 10X, and looking on the table to see where 6" MOA drop with a 300 yard zero, I came up with 525 yards for the tip of the post. This scope has very short crosshairs and long posts so thanks for the info. If I have calculated something incorrectly please let me know. Sierra 165 HPBT, 79.4g RL22, .300 Winchester Mag.@ 3100fps.
     
  8. sscoyote

    sscoyote Well-Known Member

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    You're right on. Now, go out and shoot it, and see if everything's working out right-- if not, note how many inches low/high u r at 525 yds., or set the target further out or at a shorter distance until your groups r centered on your lower post. U may have to adust mv in the ballistics program to get the actual trajectory to match up perfectly. Once the computer model matches the reticle then u should also calculate an interpolative system for the "in-between" ranges for 25 or 50 yd. intervals, like this--

    Say the ballistics program matches the (6 MOA) post tip at 525 yds. Now look at the MOA drop for say 425 yds. Suppose it says 4 MOA. If u divide 4 by 6 u get .7. This means that if the buck shows up at a lasered 425 yds., then just aim 7 tenths (or 70%) of the way down to the lower post tip from the center x-hair, and u'll kill him. Do the same for all the other ranges in 25 or 50 yd. intervals, and u will have created an interpolative ballistic reticle out of your plex reticle, that's almost as accurate as an actual ballistic reticle (the interpolative system is identical to that which is used for the mil-dot for ranging). Oh yeah-- BTW do the same for windage-- that 6 MOA is an excellent subtension for interpol. windage as well. The plex reticle is the simplest of ballistic and ranging reticles IMO-- and 1 of my favorites really. Last yr. i killed a coyote @ 300 yds. in a 10 mph x-wind using an XP-100 handgun in .17 Mach IV by applying an interpolative system for the plex reticle i was using for both windage and elevation. Had to aim .6 tenths down for elevation, and a whopping 1.4 "plex units" windage. I was as proud of that "long-range" shot as any i've ever made with a ballistic reticle.
     
  9. davewilson

    davewilson Well-Known Member

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    how about just sighting it in at 100 then see where the tip of the duplex moves the point of impact on the target when holding it on the bullseye.then you could get a pretty good idea of where it's POI would be.
     
  10. jwp475

    jwp475 Well-Known Member

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    [ QUOTE ]
    how about just sighting it in at 100 then see where the tip of the duplex moves the point of impact on the target when holding it on the bullseye.then you could get a pretty good idea of where it's POI would be.

    [/ QUOTE ]

    That is too easy........... /ubbthreads/images/graemlins/wink.gif
     
  11. sscoyote

    sscoyote Well-Known Member

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    Dave-- Do you mean put the plex post tip on the center of the 100 yd. group shooting another gp. and measuring the distance between the 2 gps.??