Elevation and changing zeros

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by birdrl, Mar 31, 2010.

  1. birdrl

    birdrl Member

    Messages:
    18
    Joined:
    Jun 4, 2007
    Ok I read the sticky on altitude and barometric pressure. I am still confused. Here is my question. How do I calculate the change in my zero if I sight in my rifle at 1000 feet elevation with standard atmospheric conditions at 100 yards and actually test my drops on my BDC reticle then go to 3000 feet elevation with standard atmospheric conditions, now my 100 yard zero is over 1 inch different and my reticle yardages are also different? Maybe I don't understand the terminology in these ballistic programs but I don't see where you can calculate what those differences will be. Thanks
     
  2. jwp475

    jwp475 Well-Known Member

    Messages:
    2,595
    Joined:
    Feb 4, 2005


    Your 100 yard zero will not change enough to matter if any at the higher alltitudes. Your drop at longer distances will be less since the air is thinner and the bullet will maintain its velocity better. A ballistics targeting soft ware program will give you the scope settings at the longer distances
     

  3. birdrl

    birdrl Member

    Messages:
    18
    Joined:
    Jun 4, 2007
    jwp475, thanks for the reply. I am not questioning your answer just trying to figure out why my 100 yard zero changes. I shoot 3 different rifles out to six hundred yards. They all have bushnell elite 6500 scopes with the BDC reticle. They are 1) 358NM 2) 338WM and 3) 338-06. All of them show a significant change in zero, +1 inch, between my sight in elevation at 1000 feet elevation and two different canyons I shoot at for distance, one at 2050 feet elevation and one at 3075 feet elevation. What else could cause that change? All three rifles are bedded and shoot very good groups consistantly. The 358 and the 338WM using Hodgdon powders don't show quite the change as the 338-06 using Reloader 15. Do you think it could be a combination of a change in atmospheric conditions and the powders. I guess I am going to have to get a Kestrel. Thanks
     
  4. jwp475

    jwp475 Well-Known Member

    Messages:
    2,595
    Joined:
    Feb 4, 2005
    Your 100 yard zero will not change enough for you to measure it is simply too close to matter
     
  5. LouBoyd

    LouBoyd Well-Known Member

    Messages:
    770
    Joined:
    Oct 15, 2007
    You've just discovered the problem with BDC knobs and reticles. They're calibrated for only one trajectory. You can use your computer programs to see the effects of air density (the combined effect of altitude, barometric pressure, temperature and humidity) to decide how far you can shoot without worrying about the change in trajectory or make up a chart of hold offs vs distance. Using a lookup table rather defeats the point of having BCD knobs or reticles, but it's no slower than using standard target knobs. The is NO simple knob setting which will correct the problem.
    Crosswind variation is normally a larger concern than drop variation from altitude changes at moderate ranges. Wind speed usually increases with elevation in typical mountainous terrain. Reduced air density however reduces the amount of bullet deflection for a given crosswind speed. Most ballistics programs show that effect too.

    The solution is to stalk closer to the game but on this forum such talk is blasphemy.
     
  6. phorwath

    phorwath Well-Known Member

    Messages:
    4,896
    Joined:
    Apr 4, 2005
    It's not blasphemy. It simply takes the fun out of long range hunting. Turns long range hunting into short range hunting. To each his own means, methods, and preferences - provided it's lawful and legal...

    A good ballistics software program will handle any environmental conditions, reticles, BDCs, and turret adjustments imaginable, under any set of differing environmental conditions. You just need to be familiar and competent with your equipment and the ballistics software prior to hunting with it.
     
  7. birdrl

    birdrl Member

    Messages:
    18
    Joined:
    Jun 4, 2007
    Thanks for your replys, it appears I have problems with my shooting technique and or scope.