What defines 'Sight Height'?

Discussion in 'Long Range Scopes and Other Optics' started by sdowney, Jul 27, 2009.

  1. sdowney

    sdowney Well-Known Member

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    I'm using a 20 MOA base. When the ballistics programs ask for sight height, are they asking for the distance between the bore and a) front b)midpoint or c) back of scope's centerline? Does it matter that much or is 1.5", which seems to be the standard, fine to use?
     
  2. AJ Peacock

    AJ Peacock Well-Known Member

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    Just measure from the midpoint of the bore (action) to the midpoint of the scope body (objective lens). I simply open the bolt until it is even with the ocular lens, then I use a caliper to measure from the middle of the bolt to the middle of the lens. Do it a couple times until you are comfortable with the number.

    Don't just use the "standard" 1.5" unless yours measures 1.5"

    AJ
     
    Last edited: Jul 28, 2009

  3. bigngreen

    bigngreen Well-Known Member

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    Some where I saw to measure the OD of your objective and the OD of the barrel add together divide by two, measure the distance between the objective and the barrel and add to the sum of previous measurement and you have the exact center to center.
     
  4. lever-hed

    lever-hed Well-Known Member

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    my thoughts also, or you could get your rings & base dimensions from the manufacturer site and use the scope tube centerline as a reference, then you can get a dim to top of receiver.. just that the bore centerline /barrel profile could be tricky .. :)
     
  5. sdowney

    sdowney Well-Known Member

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    If the scope is on a 20 MOA base, there is a substantial difference between the objective, midpoint, and ocular lens and the rifle bore. Where do you suppose I should take the measurement from?
     
  6. AJ Peacock

    AJ Peacock Well-Known Member

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    I have 2 rifles with angled bases, one is 20moa, the other is 40moa. I measured from the center of the ocular to the center of the bolt on both of them and have had no problem with the drops matching the data from JBM & Exbal.

    According to my calculations, there shouldn't be much difference between the front and the back with a 20moa base.

    1 moa at 100yds (300') = 1.0472"

    For a scope that is 1' long and 20moa base, there should only be approx .06981333" difference front to back.

    20*1.0472/300 = .069813333" (inches subtension/ft for 20moa base)

    You can measure the back and know you are within .1" of whichever number works best with your particular calculator.


    AJ
     
  7. lever-hed

    lever-hed Well-Known Member

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    Splitting hairs – lay rifle on its side, eyeball with a tape.

    Kind of a supersilious anality of an operation you ask me.. plug in 1.5 inches into the balistic calculator and draw your dope out to 1000, then plug in 1.6 , 1.7, 1.8 etc and see the differences, is it really that much? Your going to proof your dope card in the field anyway right? So any discrepancy on the sight height is going to be more then outshined by the results of firing in the field.. just my 2 cent
    …. to each their own…
     
  8. AJ Peacock

    AJ Peacock Well-Known Member

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    It is a bit anal, but it's easy to do. My LR rig measures 2.23" and the difference between that and 1.5" is nearly 1moa at 1000yds. So it does make a difference and it takes all of 1 minute to remedy.

    AJ
     
    Last edited: Jul 28, 2009
  9. lever-hed

    lever-hed Well-Known Member

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    AJ, of course there is a difference between 1.5 and 2.23, I’m talking about variances of .1, .2, or .3 inch measurements... .

    After rereading your post, I think you misread me and we are basically saying the same thing .
    My only argument was that it doesn’t have to be too exact, and if your off by .1 or .2 inches from what it really is, it won’t be the end of the world.

    ..but your wrong!!, just kidding..:D

    Good luck
     
  10. AJ Peacock

    AJ Peacock Well-Known Member

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    Lever-hed, yes, we are saying the same thing. I just open the bolt and measure from about the center of the bolt to about the center of the ocular lens. I use the handiest measurement instrument I have at hand (usually a pair of calipers).

    AJ
     
  11. Mikecr

    Mikecr Well-Known Member

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    And when you dial in or hold off, the relative centerline(crosshair) changes some microbial amount.
    Well,,, it's just a wonder we hit anything out there!
     
  12. bigngreen

    bigngreen Well-Known Member

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    For me I know my sight is 1.97 in. above the bore and when something goes weird I have a shorter list of things to look at.
    I don't want to tweak my ballistics software to make a hit, I want to input facts and get accurate come-ups and windage that I am not questioning in my mind. Anal? Yes, but it is the only way I can function and it is getting worse as I get older :D not a good sign for my poor wife.
     
  13. Jay Kyle

    Jay Kyle Well-Known Member

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    This thread is a bit old but there seems to be a lot of mystery around this measurement.

    Here's the theory - what I call the Sight Height Rule:
    With respect to ballistic software, the effective distance between the LOS (Line of Sight) and the LOF (Line of Fire) should be measured at a point where the measured muzzle velocity is valid.
    This is normally the "0" yard point with respect to your external ballistics software - this is an arbitrary datum point most folks assign to the end of the barrel. However muzzle velocity is measured at some distance away from the barrel and must therefore be corrected back to the muzzle. The problem with this method is that the pivot point for the LOS is located near the center of the scope, far behind the muzzle. So the LOS-LOF distance increases or decreases as you dial in more (or less) height on the scope. But since we sight in at 100 yards, and today's ballistic software doesn't account for more than a single scope height value we're stuck again.

    Now for the practical:
    Best to measure the sight height at the center of the scope (LOS pivot point), and correct muzzle velocity back to that point - done.
    This is the simplest and most correct way I know of to get the right values for your charts.

    Jay
     
    Last edited: Aug 17, 2009
  14. Bill Maylor

    Bill Maylor Well-Known Member

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    Ya'll can't be talking like this...CONGRESS will be wanten to take your scopes awaylightbulb and Make you go back to open sights. I like the spacer idea myself. Thats real simple. I wonder if their are any plans for KLUNKER scope trade in's.:) Bill Maylor