How much can temperature affect POI?

Discussion in 'The Basics, Starting Out' started by tlk, Jul 21, 2009.

  1. tlk

    tlk Well-Known Member

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    At 100 yards? By temperature I mean outside temp, not barrel temp.
     
  2. Michael Eichele

    Michael Eichele Well-Known Member

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    The density of the air due to temprature changes cause so little effect on POI I doubt one could accurately measure it. Changes in lighting conditions will cause WAY more POI differences at 100 yards then temprature ever will. Even out to 300 yards where I keep my rifles zeroed, I have never detected a POI change in different temps. Lighting yes, temps no.

    As you probably know, when you increase the distance, that rule is out.
     

  3. BENNYBOOBOO

    BENNYBOOBOO Well-Known Member

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    I've asked around about this before, but nobody seemed to know a reasonable answer to WHY lighting causes a change in poi. Any suggestions? Also, HOW does lighting change poi?

    EH
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    It isnt that the lighting in and of itself changes the POI, rather it changes the way YOU perceive the target.
     
  5. LRSickle

    LRSickle Well-Known Member

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    That's the first thing I wondered too. I work with some guys that shoot their old military rifles in some military fun shoots and they say the same thing. They say the direction from which the sun is shining makes a difference. I'm assuming it has to do with how the front sight is lit up and may appear to be to the right/left of where it really is. Not sure, but that's my guess. I also wonder how it could effect your aim with a scope.
    Can anybody shed some light on that?
     
  6. LRSickle

    LRSickle Well-Known Member

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    That was quick. By the time I posted my question M E beat me to the post. Thanks for the reply M E.
     
  7. trebark

    trebark Well-Known Member

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    hmmmmm.....Seems curious to say that temp would not affect POI. At 100 yards, maybe it wouldn't affect POI much, but at longer ranges I would think that temperature does have an affect. Michael, I have your ballistic program and one of the set-up criteria is temperature. Sure enough, if you put in a set of data at one temp, then change it to another (generally an large change 30 to 70 degrees), the moa adjustment does change at longer ranges.
     
  8. Michael Eichele

    Michael Eichele Well-Known Member

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    You are correct. The original post in this thread was concerning 100 yards. At 100 yards the differences are not noticable. At longer ranges yes it is and after 600 it is considerable. Out to 300 is minimal, past that it becomes noticable, but not enough to worry about in normal big game situations untill after you get past 600. Past that point, you better start compensating for it.

    Sorry if my 1st post was confusing.
     
    Last edited: Jul 21, 2009
  9. trebark

    trebark Well-Known Member

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    Michael:
    Thank you for clarifying. I guess it was too good to be true that I could forget about temperature when determining scope adjustment.

    Now this whole thing with the light...that just sounds crazy. Do I have it straight....bright light POI goes up and in low light POI is true or down?
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    I use to think so but it doesnt seem to be the case after further experiences.

    It seems to have more to do with the position of the sun in relation to you and your target. For example, if the sun is to your left and left of the target your impacts will be a bit to the left. When it is centered, impacts will be center (provided you sighted in with it in this position) and when it moves to the right, your POI will follow. I have not noticed much difference when the sun is behind me and left or right or infront of me and left or right. Right is right and left is left and center is center.

    I have noticed that when the skies are very gloomy and there is no dominating sun and zero in, it does move up or down (I cant remember which) when the sun comes out. But as far as left and right it definately changes with the position of the sun. I assume it does up and down as the sun get higher or lower but I havent been able to "test" that theory.
     
  11. BENNYBOOBOO

    BENNYBOOBOO Well-Known Member

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    Thanks for a great answer, Michael. Of course each rifle and each load are different, but what kind of changes are you seeing with the bright, unfiltered sun at say a perpendicular to your line of sight to the target? (ex: .5moa, 1moa, etc...)

    EH
     
  12. tlk

    tlk Well-Known Member

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    Here is the problem: I was trying to get a firm idea on the accuracy of a load I was working up and started shooting it in various conditions. Shot one shot two evenings - everything is good at ~.75 inches. Next day I shot in the late morning - after heavy rain the night before and a good mirage. Temperature was some higher, but the sun was hitting the barrel this time (almost sirectly above, before it had been over the horizon).

    My shots got crazy wild - like 2". Runout on the bullets was .002, Lapua brass was prepped, and the gun itself was good to go. Wind was about 5 mph with 10 gusts. All shots that day went slightly left, with two going WAY down. I cannot figure out what the hell happened. I thought I was onto a good load. I know POI wouldn't change everything that much, but I am just trying to figure out all of the variables here that could have contributed to this.

    Scope is good - had it tested, so I dont think that is it. Mounts and rings are tight. May be the shooter? All shots were in the prone position.
     
  13. Michael Eichele

    Michael Eichele Well-Known Member

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    If I understand the question correctly, The sun, myself and the target are all in the same line?

    Anyway if all three are in line, I try and zero in here. Then when the sun is to the left, my POI's are left etc... I have not really seen more than about 1/2 MOA difference. Then again, I have never fired with the sun at every possible angle.
     
  14. Frogman77

    Frogman77 Well-Known Member

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    I've found the same thing shooting bows. Indoor lighting and outdoor lighting, my poi shifts even at 20 yards. Not much, but when target precision is required it makes a difference. Even shiftng shooting lanes indoors where the overhead lights are a little to the left or right can make a difference especially at longer ranges.

    Outdoors very much like Michael's experience the sun's position left or right of my target will change my horizontal impact as well.

    Perception of the target affects the shooter's point of aim based on what direction the light is coming from. I imagine it has something to do with how the light rays are reflected off of the target back to the shooters eye relative to the angle the light is hitting the target.

    I've shot my bow out from 20 to 60 yards when sun is directly overhead. Zero'd there everything lined up centered. As the sun moved to my right, and the target's right my point of aim/poi shifted horizontally and more so as the sun approached the 3 o clock position. Which direction I don't remember. That was done last season and I haven't shot my bow in about a year due to an injury.