Slope angle shooting at Long Range.

Discussion in 'Long Range Hunting & Shooting' started by Topshot, Oct 7, 2009.

  1. Topshot

    Topshot Well-Known Member

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    Ok,

    There is something that I have been wondering about when taking long shots where some slope angle is involved. It involves the turret adjustment in MOA to compensate for the slope angle.
    If I tilt my scope at a 20 degree angle, and adjust my scope 25 MOA. Then I am getting 25 MOA of adjustment perpendicular to the slope plane, but I am only actually getting 23.5 MOA of adjustment in the vertical plane.

    Now my Exbal program will give me a corrected turret setting for the slope distance. But is this corrected setting only taking into account the slope distance or does it also take into account the fact that the scope is also now at a slope angle?
    With the scope being at an angle to the horizontal, the true vertical turret movement should always be different than if the scope was horizontal.
    Is this confusing you? because it is confusing to me.

    Eg. If I was shooting at say 800 yards, on flat country then Exbal might give me say 26.5 MOA of adjustment.
    If the 800 yard distance was down a slope of 20 degrees, then Exbal would give me a slope corrected setting of 24.75 MOA (i.e. for a slope corrected distance of 752 yards).
    Now if I punch in the same 752 yards as if it was a flat distance, then Exbal only gives me a setting of 23.5 MOA.
    So the question is, where did the extra 1.25 MOA of adjustment come from? Is it a correction for the scope slope?

    If this is the case then using an angle cosine indicator and simply reducing the slope distance to the horizontal in the field is sure to give you incorrect turret settings for most long shots at any sort of steep angle.
     
  2. 7mmRHB

    7mmRHB Well-Known Member

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    Topshot, even though the 800 yd 20* sloped shot is corrected to 752 yds because of gravity effect, the bullet is still traveling 800 yds with the effects of air drag and time of flight.

    The flat 752 yd shot has less time of flight and less time for the air drag to effect it's drop.

    I may not have the terminology correct but I think it is the basis for what you've noticed in the Exbal programing. ---- RHB
     

  3. Bravo 4

    Bravo 4 Well-Known Member

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    +1, The bullet will still have the time of flight as with a 800yard shot and so drag and wind will affect it accordingly.
     
  4. pyroducksx3

    pyroducksx3 Well-Known Member

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    I was thinking of a problem similiar to this. Lets take the 800 yard shot. it is corrected to 752 yards as stated. How about windage. Do you figure your windage off the corrected 752 or the 800. By using the logic above I assume since it still travels the 800 yards you would use that distance to determine the windage and not the corrected 752, right.
     
    Last edited: Oct 8, 2009
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    Lets not forget that the MOA value/inch relationship is different from 752 yards versus 800 yards.

    EXAMPLE 1 MOA at 752 yards = 7.87"

    1 MOA at 800 yards = 8.38"

    Say a bullet trajectory is 124" at 800 yards at 0 degrees. If you add a 20 degree slope and it has a 12" cut it falls 112" @ 800 yards. The bullet still travels 800 yards even though it acts like it travels less. For your scope to work properly you still compensate for 112" at 800 yards. If you use the yardage it acts like on level ground youre screwed. This is where the extra 1.25 MOA is coming from.

    Clear as mud??
     
  6. Topshot

    Topshot Well-Known Member

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    So if I take my scope off my rifle and put it on the kitchen table. Tilt the scope 20 degrees and dial up say 25 MOA. What do I get?

    I get 25 MOA of adjustment in the same plane as the scope but only 23.5 MOA in the Vertical plane.

    So the way I look at it. A field tactical shooter who is using a simple angle cosine indicator and reducing the slope distance back to a horizontal distance. Then treating the shot as a simple flat 752 yard shot should miss the target low due to the error in not taking the slope of the scope etc into account.

    Am I barking up the wrong tree on this?
     
  7. Michael Eichele

    Michael Eichele Well-Known Member

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    Yes.

    You cant treat it as a 752 yard shot.

    Lets say a given bullet drops 152" at 800 yards and 124 @ 750 yards.

    Then add the 30 degree slope. 800 becomes 124" of drop. Yes the drop in inches is the same as 750 yards on a level plane. However if you treat it as a 750 yard shot using MOA youre going to be off. 124" at 750 yards = 15.75 MOA. 124" at 800 yards = 14.75 MOA regardless of slope. The inches of drop is the same between the two yes. The MOA used to compensate is NOT the same. The bullet's path changes due to the slope as you know. Those changes are not compatible with any distance other than the line of sight to target. You still have to figure your MOA from the inches of drop at the range fired. The MOA to inches of drop relationship is different for every range even if the inches of drop was the same from range to range.

    I understand what youre saying about the scope being less than 25 MOA on the vertical plane when adjusted to 25 MOA on a slope not 0 degrees. This is not the issue. Regardless of slope, when youre looking at a target at 800 yards line of sight your scope when moved 25 MOA will move the reticle 25 MOA at 800 yards in relation to the target regardless of where the bullet impacts. When the bullet impacts at less drop than normal due to this slope you compensate accordingly. When the bullet falls 152" at 800 yards the appropriate MOA is 18.25. When it falls 124" the appropriate MOA becomes 14.75. The slope doesnt matter at this point. The slope dictates a bullet trajectory. Even though the slope will cause a bullet to drop less than a shot at 0 degrees the line of sight distance is NOT null and void for calculations. Line of sight is line of sight, inches are inches and the MOA at range is MOA at range.

    I hope that helps.
     
  8. Topshot

    Topshot Well-Known Member

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    Thanks Michael,
    It is a complicated thing made even more complicated by not knowing exactly what goes into these Ballistic programs to calculate their solutions.
    I think I need to get some graph paper, draw some curves and triangles, and do some math.
     
  9. Michael Eichele

    Michael Eichele Well-Known Member

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    I cannot speak for other calculator solutions but both of my calculator solutions are as follows:

    I use the cosine of the degree * Pi / 180 * true drop at range (from a level bore) - the corrected drop from zero.

    This finds the inches of drop at the line of sight range.

    So the cosine of 30 degrees is .15 * PI / 180 = .866

    Take .866 and multiply this by the drop at line of sight range from a level bore. For my favorite load that is 187" at 800 yards even though my impact will be 124" due to a 300 yard zero. For now we are only concerned with the true drop from a level bore.

    187" * .866 = 162". Now deduct 162" from 187" = 25". Now deduct 25" from your drop at line of sight from your corrected for zero impact of 124". This equals 99" of drop from zero at 800 yards.

    As far as finding the MOA for correction you can take 99" / click value in inches / number of clicks to make 1 MOA.

    So for a standard .25 MOA scope one click = .26175" * 8 (which is yardage / 100) = 2.094" per click at 800 yards / 4 (4 clicks per 1 MOA) This = 11.82 MOA where as a level shot would be 124" or 14.8 MOA

    Like I said, I cannot vouch for how exbal, RSI, Sierra etc.....arrive at the numbers that they do. However my programs and other program solutions match very very closely.
     
    Last edited: Oct 9, 2009