Need advice: drop at different elevations

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by botill, Apr 27, 2010.

  1. botill

    botill Member

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    I will be going on an Elk hunt in Wyoming this October and am getting prepared. Can never start too early right? :)

    I enjoy and practice shooting often and am very confident in my ability out to 600 yards in field conditions. However, being in Texas, I practice at around 500-1500 feet in elevation and will not have the opportunity to practice or test at the elevations I will be hunting.

    My question is this: Assuming a long shot opportunity at 0 degrees, can i simply use the data from the ballistic calculators and trust the adjustment information it provides for higher elevations or are there variables other than physics that must be factored?

    Advice from those which have experience with this is much appreciated!
     
  2. Buffalobob

    Buffalobob Writers Guild

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    As long as you have properly calibrated you drop chart in Texas to local conditions then a simple elevation change will not mess things up. You will need to spend a few minutes to check you zero after getting to Wyoming being as not all rifles travel well.
     

  3. papa45

    papa45 Well-Known Member

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    Buffalobob,

    I don't mean to argue with you, but I don't understand your answer. Presuming no change in temperature and muzzle velocity, trajectory is a bit flatter at higher elevations. A simple ballistics program I use says, going from 1000 ft to 6000 ft MSL, the POI changes about 5 inches at 600 yards, for a 180 gr, .30 cal bullet at 3000 fps MV. Am I missing something?

    I've made myself crazy thinking about this for some of my own hunts. For most powders, colder temperature means lower velocity. As a practical matter, going from Texas to Wyoming (higher elevation, lower temperature) the two changes tend to cancel each other out and the net change becomes less significant. (Presuming no changes in barrel harmonics.)
     
  4. Loner

    Loner Well-Known Member

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    Temp. and alt. change the real world bc of any bullet. So alt. or barometric pressure
    can be used to adjust your calcs. Most programs will cancel out one when the other is
    imputed. Higher humidity will increase the bc slightly but I don't usually see enough of a
    change to worry about it. None the less your program should have a humidity field
    as well.
     
  5. Buffalobob

    Buffalobob Writers Guild

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    Well OK. If you want the long detailed answer then I guess I can type more than two sentences.

    While in Texas you should calibrate your drop chart based upon your muzzle velocity and BC at your local elevation. You have to take into account temperature, humidity, etc in order to determine a correct for Texas drop chart.

    You will need to use one of the ballistic computer programs such as Exbal or JBM

    JBM - Calculations - Trajectory


    Once you have everything calculating satisfactory for Texas and you get to Wyoming you will need to change the inputs according to the weather and elevation at which you hunt if you have a PDA based computer program. Obviously the temperature is going to change during a day and day by day. It may be blowing snow one day Coppertone time the next.

    If you do not have a PDA with a ballistics program and you are going to limit your shots to 600 yards then you can built yourself one or more drop charts that will allow you to hit an elk easily. They are pretty big!

    What you should do is use Google Earth to find the average elevation at which you think you will be hunting and the minimum and maximum. Then find out what the early morning temperatures are likely to be for those elevations and what the average daily temperature will be.

    So for each elevation you should print out a drop chart for the morning temperature and the average daily temperature.

    Once you have all of these drop chart printed out you can then find out how much a 10 degree change in temperature makes in bullet strike at 600 yards. You can also figure out how much a 1000 foot change in elevation will make to bullet strike.

    At the top of my drop charts I have the temperature and elevation for which it was calculated and then the amount of change that will occur at 1000 yards for a ten degree temperature shift and the amount of change for 1000 ft of elevation change. This way I can move around during the day and still work off of one drop chart. This same technique is good for BDC knobs.
     
  6. botill

    botill Member

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    BuffaloBob,

    That is exactly what I was looking to confirm. Thank you and the others for taking the time to respond.

    I run a ballistics program on an iphone which works very well and will have it handy when I go in case needed.

    Thanks again!