how does time of day effect 1000yd shot

Discussion in 'Long Range Hunting & Shooting' started by malcarjeb, Dec 20, 2010.

  1. malcarjeb

    malcarjeb Well-Known Member

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    I sighted in my rifle this afternoon at 1000yards and was wondering how shooting in the morning time vs the afternoon would effect the point of impact? I know there is a benchrest saying "lights up sights up" and I find this to be true, the later it got(setting sun,lights down) the bullet seemed to stay up better, taking less clicks that i though it should. I have never shot in the early morning time to see how it would effect the bullet. I have a hunting trip in the morning for whitetail deer and was looking for some advise on this. Thanks for your help.
     
  2. elkstalker300

    elkstalker300 Well-Known Member

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    The only thing I would figure that would mees with the dope would be temp in the morning is cooler then afternoon, maybe baro pressure, get your ballistic calc out and mess with all being the same except temps and you will find the warmer the temps the hotter your round will shoot, try it out and see
     

  3. royinidaho

    royinidaho Writers Guild

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    I would suggest that the less dense the air (warmer) the higher the bc the higher the point of impact impact with the same point of aim.

    Additionally, the higher the temperature difference between the ground and the air, mirage w/no wind, the higher the point of impact with the same point of aim.
     
  4. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Malcarjeb,

    The saying "lights up, sights up" isn't a benchrest saying; it's from Highpower competition, and applies more specifically to Service Rifles, since they use a post front sight. The phenomenon that brings this about is actually an optical illusion, and not related to ballistics per se. In this type of shooting, most competitors use a six o'clock hold on the target, resting the bottom of the bull on the top of the front post. In periods of brighter sunlight, the target appears larger to the eye, and that same hold is now actually slightly lower on target; you're holding lower than you did in darker conditions with the same sight picture. As a result, when the light goes up, the shooter generally needs to take a click or two up, to wind up with the same POI on target. When the light goes down (gets darker), you need to take a click or two dow, for exactly the same reason in the other direction. Strictly an optical illusion, but it does effect your point of impact. This isn't a problem with the aperature front sights used on most Match Rifles, or scoped rifles.

    There is no specific number of clicks to account for these light changes, and not all shooters are affected to the same degree. Some see no change at all, others need to pay very close attention to it. All a matter of personal visual accuity, I'd guess.

    Just wanted to clear this up, hope it helps.
     
  5. royinidaho

    royinidaho Writers Guild

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    Kevin,

    You are a well of knowledge!

    Thanks

    Roy
     
  6. Kevin Thomas

    Kevin Thomas Well-Known Member

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    No, just an avid Service Rifle shooter for the past thirty years or so, who has only recently started to dabble with the wonderful world of aperature front sights and match rifles! Don't have to worry nearly so much about the old "lights up, sights up" when using these, and that's a good thing. :)
     
  7. RMulhern

    RMulhern Well-Known Member

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    Kevin

    As a Distinguished Rifleman...member of President's Hundred, Palma 20, and former member of USAMTU, etc......it's damn nice to read something from someone who knows about which they are speaking!

    MERRY CHRISTMAS to you sir! I always enjoyed my conversations with you when you were with Sierra!!
     
  8. SBruce

    SBruce Well-Known Member

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    When shooting a new custom 338 edge on paper at 950 yds recently.......we noticed a change in point of impact from day to day (bright/sunny/warm vs cloudy and slightly cooler) Temps, BP, RH etc. were accounted for in the ballistic program, but poi was still noticably higher on the bright sunny days. Perhaps it was due to Mirage boil more so than sunlight?? Too little experience with same rifle/load at same distance to say for sure.

    We are still uncertain how much of this was a scope tracking issue, because the scope seems to change zero by at least 1/2" at 100 yds (more testing needed to absolutely confirm). But in my limited experiece at 950 yds, I'd say aim slightly lower on bright sunny days, But don't quote me on this. Just throwing in my .02 (and it might not be worth that much)! Still learning stuff myself when it comes to the 1000 yd mark.