Importance of Sight Height in Ballistic Calculations

This is a discussion on Importance of Sight Height in Ballistic Calculations within the Rifles forums, part of the Sniping Related category; I've always had some trouble getting my actual drop tables to agree with the results from ballistic programs. This is particularly true for the 500-700yd ...

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Thread: Importance of Sight Height in Ballistic Calculations

  1. #1
    Senior Member gsmithplm's Avatar
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    Importance of Sight Height in Ballistic Calculations

    I've always had some trouble getting my actual drop tables to agree with the results from ballistic programs. This is particularly true for the 500-700yd range. Since I rather like the Ballistic AE program for my iPod, I decided to email Johnathan Zdziarski (the author) about the problem. He suggested checking a number of different things, one of which was sight height.

    There are numerous articles about how to measure this and I think that I must have tried everyone of them last night, and even invented one of my own using a bore guide and cleaning rod arrangement. The long and short of this is that it is possible to measure several times (with calipers) and get slightly different readings due to the rather awkward nature of the components.

    So I started playing with sight height in the program to see what effect that had and was extremely surprised at a small change could make a perceptible difference, particularly in the very region I was having the problem with. Since many people accept the default on this, it could be that this is something people might want to spend more time looking at.
    Graham Smith, SFC, US Army (Ret)
    "There's no right way to do the wrong thing."

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    Member L-Dogg's Avatar
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    Right on Brother......damn good point I reckon. t makes sense and in my case I battled to get an accurate sight height reading......and by accurate I mean dead on. I chose 1.5" as it seemed the closest but it's all eyeball really. I'm going to play around and see if my numbers make sense when altering the sight height by small amounts. I never thought of trying that. What method did you come up with using the bore guide/rod etc?

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    OS1(SW) GMinor's Avatar
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    Agree. Playing around with it, if it is off by .2" then I see about a .1-.2 mil difference from 500-700 yards.

    I have always measured this though, kinda thought it was standard practice.
    Last edited by GMinor; 06-26-2013 at 09:55 AM.

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    Member L-Dogg's Avatar
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    Using my zero data for 100Y and based on data for a sight height of 1.5"....and then changing the sight height to 1.9" results in a difference of about 2.5"to 3" at 800Y.....according to my crappy math. What did you come up with for your setup?

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    Member L-Dogg's Avatar
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    Turns out I was missing an easy way all along. My bolt has several holes along the length that are on the centerline. My scope has a short manufacturer hash mark on the centerline at the windage knob location.

    My scope is not at 1.5" above the bore as I entered it in BAE.

    It is at 1.9" above the bore, taking readings at that location....midway in the ejection port. If that's an okay place to measure it?

    I recently shot at 800, 900 and 1000Y....and, apart from the problems I was having with myself........ ......... the elevation issues I was having had much more to do with actual velocity out at that distance, than the incorrect scope height I chose to input......as regards the numbers in Ballistics AE. Being only a 20" barrel and 2595 MV. After I got it all sorted using the truing function, the day ended well.

    But in general, correcting this .4" error can only help. After all......this is supposed to be a precision game.

    Thanks for bringing this up in this thread. In and amongst all the angst when my numbers seemed quite a bit off at 800-1000Y....yet were perfect at 200-600Y.....it occurred to me the sight height number I entered could have had something to do with it. I was meaning to suss it out and it just slipped my mind completely. This thread prompted me to get the rifle out and figure it out.....and not guess. Whether or not it makes a difference, big or small.......I can now take that doubt out of the equation. I wish I had corrected that before I traveled all that way to go shoot out to 1000Y for the first time with this rifle. You live and learn.

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    Senior Member gsmithplm's Avatar
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    The main reason that I never looked at this is that it's almost never mentioned as being all that important. I still have not found a way that I really like for measuring this but I'm getting closer. The main problem is that my bullet measuring calipers can't get in close enough, but it occurred to me last night that I have some old drafting tools somewhere and in there is a compass which should work for me.

    This whole thing has taken on a life of it's own. I like puzzles and problem solving and I'm determined to get this as close as possible to reality and, in the process, learn what tweaking various values will do to the end result.
    Graham Smith, SFC, US Army (Ret)
    "There's no right way to do the wrong thing."

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    OS1(SW) GMinor's Avatar
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    I don't think it is necessary to be hyper accurate in your measuring. Running the numbers on BAE from 1.7" to 1.9" sight height I get about an inch to an inch and a half difference in drop at 1000 yards. All of my .1mil click values remain the same.

    Using the default 1.5" vs my actual ~1.8 results in pretty much the same thing. Same click value at 1000 yards 1.5 to 1.8.

    For your ranges of 500-700 the biggest difference I have seen going from 1.5 to 1.9 sight height is .04 mills.

    Is .04 mills at those ranges really worth making sure you are within the 0.0x range of measurement? No. A ruler will get you close enough for the difference to be insignificant.
    hoagymaster likes this.

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    Senior Member gsmithplm's Avatar
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    I rather suspect that it depends on the bullet and load. With my load, the difference between 1.5 and 2.0 sight height is most pronounced at 600yds and 700yd, where there is a 0.2mil difference.

    Granted, the difference between 1.7 and 1.9 is probably less significant. But, at this point it's mainly become a game to see if I can learn enough that I can fiddle with the numbers and get as close as possible to actual. Then I just need to go someplace with completely different weather than what I've already done and see how well it works. Not so easy when you live where I do.
    Graham Smith, SFC, US Army (Ret)
    "There's no right way to do the wrong thing."

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    On my rifle, when I changed rings, my sight height went from 1.7" to 1.9" . At 1000 yards that .2" sight height change made about 2 " difference in bullet impact. Impact was 2" higher with the higher rings.

    Sincerely,
    Dave (Bubba) Thornblom

  10. #10
    Member L-Dogg's Avatar
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    Yeah...it seems it depends on how hyper accurate you want to be. If you call the wind right, this 1"to 1.5" discrepancy in elevation at 1000Y, could be the difference in not hitting the X ring unless you hold for it. But you are right GMinor....... it is really nitpicking at the end of the day...especially shooting at plates. BTW........howcome you didn't go for GMinor7th(b5,b9) ......much more hip.

    Graham
    ......I read somewhere as I am sure you did, that the shelf of the stock at the ejection port is usually at about the centerline of the bolt and it is easy to rest a ruler on there and measure to the center of the windage knob. This get's you close enough they say.

    Personally I am happy with a value of 1.9" and will just stick with that now and stop hyper over-thinking it. Something I am trying hard to stop myself doing, in general, as it is a bad habit of mine. My plan now for the foreseeable future is to just leave this gun alone and just shoot it. I have done pretty much all I can do and it is shooting to the limit of it's potential as a stock factory action. I'm sticking with one long range load I have zero'd to....175gr SMK and 43.5gr Varget and that is it for a while.

    Would this type of caliper/compass work better for you....help get around things? I'd actually love to have an old vintage one like that....just to have. Yard sale material maybe.



    il_fullxfull.364027725_4rwx.jpg

    The only question I have that does not seem to get answered 100% to my satisfaction is.......at what point along the barrel or beyond it......should the sight height be determined.....or does it not matter? And what about having a 20MOA or greater canted base?

    But these are just questions now and info for the next one. I'm happy now.....and don't think these minute discrepancies will make a difference. .4" yes! .1 or less.....not enough to sweat it.

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