Do you need to rezero a scope if you significantly change elevation?

Discussion in 'Long Range Hunting & Shooting' started by teddy12b, Dec 18, 2007.

  1. teddy12b

    teddy12b Well-Known Member

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    I'm from Indiana and I was wondering about this the other day.

    If I get all set up and know how many clicks on my scope to make accurate shots at different ranges and then go out west to hunt at 9,000 ft elevation or something similar would I have to start all over again?

    I know that elevation effects how a bullet flies so I'm thinking I'd have to find a place to re-zero my rifle before any shots were fired. Even if I knew how many clicks of adjustment on my scope I would need in Indiana it wouldn't be 100% correct when the altitude changes.

    Am I right on this? If I go hunting out west I'd need to get to a range first and check everything, or would I be good to go?
     
  2. lerch

    lerch <strong>SPONSOR</strong>

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    While the 100yd zero wont really be affected you would be amazed what a big elevation change will do past 400yds or so. A good ballistics program like Exbal will get you real close but its always nice to run a drop chart and then compare it to some real world drops at extended range.

    steve
     

  3. goodgrouper

    goodgrouper Well-Known Member

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    Luckily, MOA is always constant and so is IPHY. So no matter what elevation you're at, if you move your scope 4 clicks, it will move the reticle 1 inch at 100 yards (if the scope is a .25" IPHY scope) if the scope is working as it should. In other words, the internals of the scope are not affected by elevation change.

    However, your external ballistics change significantly with altitude change. Bullets fly through the air better at higher elevations. So the key is to have your rifle properly bedded and the barrel free floated so that the point of impact remains the same and functions similiarly to the scope in that it does the same thing every time. This ensures that there won't be impact change due to equipment.

    At 100 yards, I have never seen a significant change in trajectory at upper altitudes. It WILL BE THERE but most guns can not shoot accurately enough to detect it. It may be less than a bullet's radius. But when you add time to the trajectory (or shoot farther distances in other words) the differences become more apparent. When shooting at excesses of 11,000 feet, I may see minutes of difference in trajectory in shots of 1/2 mile and farther depending on the particular bullet. But POI at 100 yards usually stays within several thousandths of where it was at lower elevations.

    So, what I do is make my gun hit at say 3" high at 100 yards at my home elevation. Then run a ballistic chart and enter this information. It will then spit out at "zero" yardage. Let's say it ends up being a zero of 310 yards. Then if I decide to go up to 10,000 feet, I simply run the chart with my new elevation but reconfigure my zero and adapt the chart for the new zero which may be now 320 yards while my 3" high poi at 100 yards stays the same. If you don't reconfigure your zero and keep your poi the same at 100, the program will automatically readjust the entire parabola before the zero AND after the zero. Or in other words, it will keep the 310 yard zero but it will change the POI at 100 yards to something like 2.8" and your gun will not shoot .2" LOWER at 100 yards at higher elevation obviously. If anything, it will shoot .2" higher at higher altitudes.
    So the computer will give you false information if you let it.

    Clear as mud?
     
    Last edited: Dec 19, 2007
  4. WSmitty01

    WSmitty01 Well-Known Member

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    I "zero" my rifle at sea level here in Florida and hunt at 8,000-9,000' out in New Mexico. I run a ballistics program for the ammunition I carry that allows you to plug in altitude. I use 8,500' and print the results and tape it to my butt stock. There is a significant difference at longer ranges!
     
  5. RBrowning

    RBrowning Well-Known Member

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    Altitude effect on MV?

    Does anyone know of any research on the effect of altitude on the burn rate of the powder? I know that the bullet will fly through the thinner air better, but there isn’t as much oxygen available for the powder to burn with. Any one ever test MV at two altitudes with the same lot of ammo at the same temp? I wonder how much your MV may decrease.
     
  6. davewilson

    davewilson Well-Known Member

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    why isn't there as much oxygen? the density of the oxygen will be at whatever altitude you loaded your shells at. this is provided they are sealed up the way they should be.

    to answer the original question. i have a drop chart made at around 1000 ft. i make changes for distance, altitude and barometric pressure.the information for all 3 is on the chart.

    i know a guy that's long range hunted for at least 35 yrs. said he shot at an elk at 2200 and his spotter said you're hitting way high. his drop chart was based off of our altitude back here and he was shooting at 9500 i think. i plugged in his numbers and the program said he would be around 50 ft high. that's right, i said 50 ft, not inches. altitude can make a big difference.
     
  7. goodgrouper

    goodgrouper Well-Known Member

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    Yep. It doesn't make a bit of difference. The internal ballistics stay the same. I have chronoed the same gun at 5000' and 10000' and the sweet spot stayed the same, gave the same accuracy, same speeds, same deviation.

    Some short range benchrest shooters pay special attention to humidity and density altitude in relation to loads but that is mosltly because we are loading at the range and our powder sits in a bottle all day long in the great outdoors. The lower Oxygen levels at higher elevations do not factor in. Actually, oxygen levels anywhere don't factor in as powder will burn without oxygen.
     
    Last edited: Dec 19, 2007