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Thread: How much does temperature and/or humidity change affect point of impact?

  1. #11
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    Thanks for all the replies. I'll shooting it a little more and see how things shake out.

    NYH1.

    Take nothing I say personal, remember....it's just the interweb!
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  2. #12
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    Quote Originally Posted by Send it... View Post
    Actually, thats not quite correct. Exactly opposite.


    https://gundigest.com/more/how-to/tr...let-trajectory



    “Well, contrary to common thought, higher humidity results in thinner air. Yes, you read that right. Despite how it feels to walk out into high humidity (the air feels “thicker”), the air is actually less dense. Therefore, it has the opposite ballistic effect that you might expect — a bullet travels easier through humid air.”
    I stand corrected. Thanks Send it.

  3. #13
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    You can learn a lot by just plugging different variables into a good ballistic calculator like Applied Ballistics or JBM.

    Here are some quick ones I ran up in AB, based on a 175gr SMK at 2650, zeroed at 100 meters.

    0% RH to 100% RH will change POI at 1,000 meters by 3.29 inches (526.42 vs 523.13 inches of drop, humidity actually reduces drag due to the size of water molecules in the air).
    So, humidity is only a minor factor. When working up a ballistic profile, I just set it at 50%.

    0 Degrees F to 100 Degrees F will change POI at 1,000 by 88.22 inches (580.77 vs 492.55 inches of drop). Temperature is a significant factor.

    25.69inHg vs 32.00inHg (lowest recorded vs highest recorded barometric pressure) will change POI at 1,000 by 103.38 inches (561.84 vs 458.46 inches of drop). Barometric pressure is a significant factor.

    However, barometric pressure and temperature work together (along with the little kicker from humidity), and are best expressed as Density Altitude (DA), which I monitor on a handheld for constant real-time update to my ballistic profile. Well, I say monitor, but the Kestrel with AB does it for me, which is super cool and makes life easy.

    But anyway, yes, environmental variables could shift your groups at 100, and that's without even getting into powder temp and other mechanical variables.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  4. #14
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    So I guess a question I have is, should I adjust my zero and bring the groups down .5" to .75" or just keep shooting it and see how the temperature change in the fall affects it?

    Thanks, NYH1.

    Take nothing I say personal, remember....it's just the interweb!
    ROLLTIDE!
    Fair Winds and Following Seas AC & IG!
    New Yorkistan sucks.
    NRA, NYSRPA, S.C.O.P.E. Member.
    FUAC

  5. #15
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    Quote Originally Posted by NYH1 View Post
    So I guess a question I have is, should I adjust my zero and bring the groups down .5" to .75" or just keep shooting it and see how the temperature change in the fall affects it?
    All depends on how much of what kind of shooing that you are doing, and the overall purpose of the rifle.
    When I'm first getting a precision gun set up I aggressively track my zero to see what different environmental conditions are doing, and how the gun is performing.
    Precision practical performance hinges on the zero.
    If you really care about being able to get real-world effect, I believe that you need to have three things: a decent laser range finder, a good chronograph (such as a MagnetoSpeed), and a Kestrel with Applied Ballistics in it; and use them. Back-up confirmed information in a data book or shooter diary for reference, and keep that information handy.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  6. #16
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    Big D ,a member here told me one time when out doing long range shooting the high humidity would cause our long range hits to be higher. I would have thought just the opposite. He is rarely wrong n my experience.

    PB
    "Air Force / Policeman / Fireman / Man of God / Friend of mine / R.I.P. Steve Lamy"

  7. #17
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    I dunno how much one would notice this shooting with iron sights at only 100 yds, but this sure sounds like what happened to me the other day.

    Very hot, humid weather, shooting XM193 through an iron sighted 14.5" carbine that had its zero verified only a week or so before. Gun is consistently hitting approximately 2" high. Groups range from unremarkable to pretty good (relatively speaking of course, this is xm193 being fired using iron sights and a shooter with poor eyesight) but everything is just high, high, high. A2 rear sight elevation at the same setting I always keep it on.

    I had attributed this to either me just having an off day or the weather affecting me somehow. But perhaps the heat and humidity was actually affecting the trajectory of the bullets?
    Too be fair it was a different lot of XM193 than I had previously zeroed with, but I've gone through a shit ton of this stuff and havent noticed that level of variation before.
    Last edited by Circle_10; 08-03-19 at 07:59.

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