I dont buy this dope.. Not the drugs either !!

Discussion in 'General Discussion' started by Down Under Hunter, Jan 1, 2008.

  1. Down Under Hunter

    Down Under Hunter Well-Known Member

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    Just in from a session with my edge and cant work a problem out.

    This afternoon was extremely hot between 93.5- 102 f.

    conditions were

    Temp 99.5
    Alt 569 ft
    BP 29.3
    RH 33.5

    At my 415 gong that I was only grouping just high on with 7.5 up,a few days ago I shot 2 x 2 shot groups 10.5 " high ??

    So I revalidated the data and got some crazy predicted velocty number of 3029 fps for the 300 smk ?? No way I say.

    Anyway, used this reading for the 700 hold over and got 14 moa up.

    4 out of 5 shots all 9 " high of centre with the fifth dropping into the bull.

    Now keep in mind that for the same target only a few days earlier I used 16.75 at was dead on for drop. Now using less ups, I'm well high and I,m wondering if it could be contributed to the extreme heat or maybe some sort of mirage effect that exbal does not factor ??

    Also I got to keep in mind that I just shot the 3 shot group of my life at 700 M -1.75 " accross and .75 verticle. Never seen one like that before, so something is sure working right !!

    Just fairly new to this and looking for clues.

    By the end of the 10 shots, I re validated the data and now have a predicted velocity of around 2867. This seems a bit more like it.

    I'm guessing that the reason I'm high is not because of exbal or shooter error, more like the extreme temp. At least I hope so.

    Has anyone had like instances when shooting in such temps and how much will mirage effect the poi ??

    Cheers all

    DUH
     
  2. Buffalobob

    Buffalobob Writers Guild

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    My experience with those kind of tempratures is that things go bad quickly. A top end load will be OK up to about 85 degrees F and then pressures get high very quickly as temperatures increases. Three things cause problems as best I can determine from just casual shooting (no instruments to do scientific measurements).

    First is if the cartridge itself is allowed to sit in the sun and get hot. I keep mine in a cooler and that is about 50 degrees F. Not cold and not hot.

    Secondly, is if the barrel is subjected to both the ambient temperature of air and direct bright sunlight it will get hot on its own and not cool very well and will stay hot. Heat is then transfered to the cartridge and things get hot.

    Thirdly is the firing rate may heat up the barrel and the inside will be hotter than the outside. I try to limit the time a cartridge stays in a hot or warm chamber to about 15 seconds on those days when I am too impatient to let the barrel cool.


    P.S.

    Now you know why I advised you to reverify the rifle zero every day that you go and develop your drop chart. You might not know why things are different but at least you have a constant known zero. All of this work is what separates the men from the boys.
     

  3. Down Under Hunter

    Down Under Hunter Well-Known Member

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    Bob,

    Thanks for your thoughts. I think in future I will just ditch shooting in these conditions unless I'm hunting and have too ?

    I had to wait a long time between shots and also had to place the gun in shade to cool with a damp rag over the barrel. I'm sure the rifle was never dead cold at any stage though.

    I think you are on the money because my first shot at 340 m was dead on ? So I guess as soon as this shot was fired, plus the extreme conditions changed things a little ?

    So I take what your saying that due to the high temps, the pressures would have been increased greatly, thus causing the trajectory change ?

    Also the ammo would have been warm from the direct sun.

    Many thanks Bob !
     
  4. Buffalobob

    Buffalobob Writers Guild

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    Having pondered things a little more, I would also consider that this is a new rifle and scope mounting. I would get a screwdriver and the allen wrenches and check everything just to be sure. You probably did that but sometimes we get upset about a bad day and forget the basics.

    But my experience is that bad things happen at the temperatures you were shooting at if you can't keep things cool. It is not just the trajectory, but the whole dynamics of the rifle gets bad. Others can say what they want, but I have just had too many bad times at high temperatures.

    There is a thread on the 115DTAC where a guy quotes David Tubbs as saying a hot barrel causes bullets to blow up. This I believe personally from my own experience but did not know anyone else believed it being as everyone blames it on barrel twist. So, I conclude things just go wrong at high temepratures.
     
  5. Down Under Hunter

    Down Under Hunter Well-Known Member

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    After speaking with a friend who has f class experience he puts it down to mirage. I never knew what mirage would do until he explained it to me.

    Shooting 1000 yards f class, he can go from 22 moa up to 18 moa when mirage is bad.

    All just learning as I go. Now I know what can happen.

    Cheers Bob !

    DUH
     
  6. Stormrider

    Stormrider Well-Known Member

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    Here is something I do when I want the barrel to cool faster in hot temperatures. I take cotton balls saturated with rubbing alcohol and rub down the barrel. Cools it off right smartly by evaporative cooling and there is no moisture left over.