Accuracy change with Altitude change.

Discussion in 'Long Range Hunting & Shooting' started by porkchop401, Nov 13, 2012.

  1. porkchop401

    porkchop401 Well-Known Member

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    Fellas , my home address is at around 200 feet above sea level in louisiana where I worked up the hunting load for a elk hunting trip in Colorado where elevation was adout 9500'. Accuracy was averaging 3/4" @ 100yds. After rangeing testing the load out to 600 yards and adjusting the numbers for altitude and average temperature range I felt I was ready to go !! . After arriving for the hunt I went to the local dump/ shooting range where I shot a group that measured a little over an inch , so I shot another and it was just about and inch. After checking every thing I could not find a thing loose . After returning home I check the zero and after a cleaning the rifle shoots at its previous 3/4" grouping . The rifle was clean when shot in Colorado. I shot both sets of groups from my portable bench whitch I took with me . I am at a lose here , Is it possible for a load to be affected in terms of acccuracy aside from trajectory due to altitude and or temperature? thanks PC401
     
  2. orkan

    orkan Well-Known Member

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    A 1/4" deviation in accuracy is not something that is easy to identify. Point in fact, I've only met 2 people in my entire life that were consistent 1/4 moa shooters.

    Having an "off day" can cause much more than that. Going from 200ft to over 9,000ft is known to put someone "off" pretty damn easily.
     

  3. webs

    webs Well-Known Member

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    It could even be from you breathing heavier in the thinner air.
     
  4. 4xforfun

    4xforfun Well-Known Member

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    I agree....altitude would have no effect on accuracy. Your accuracy will change if you change the INTERIOR BALISTICS, not the EXTERIOR BALISTICS.

    Tod
     
  5. porkchop401

    porkchop401 Well-Known Member

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    Well fellas , I think that it could be just me as well, I suppose that the stress of wanting to maintain accuracy while on a serious hunting could shew my normal shooting ability. lightbulbI appreciate your thoughts! pc401
     
  6. BPreheim

    BPreheim Member

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    Tempture, humidity, altitude,(barometric pressure) will all affect group size 1/4 in change in size is well within the norm. Some powders are much more temp sensitive and velocity change can be significant witch will affect accracy.
     
  7. 4xforfun

    4xforfun Well-Known Member

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    Hogwash!!! You are talking about interior balistics. If the velosity is the same, the group size is the same. You are talking about reducing/increasing velocity due to temp sensitivity of the powder. the only thing that does that is INTERIOR balistics. Change the velosity and of course the group size could get bigger....but..it could also get SMALLER. It just depends on how carefull and anal you were during load development. Baro, altitude, humidity have ZERO affect on accuracy. Temp also has zero effect on accuracy, provided that you either keep your ammo the same temp as your load development or you tweek your velosity to keep it constant with your load development.
     
  8. chucknbach

    chucknbach Well-Known Member

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    Once the bullet leaves the muzzle it is under the influence of external forces. To say it has no bearing on accuracy is hogwash. 1/4" at a hundred worth, I don't know.
     
  9. 4xforfun

    4xforfun Well-Known Member

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    Of course outside or exterior balistics have a bearing on the flight of a bullet. POI can change drasticly due to temp, Baro,ect....wind can blow it all over hell, but the ACCURACY will not change. If you can shoot a 5 inch group at 1k in calm conditions, you can do the same thing in a 20 MPH wind if it stays constant....If you can shoot a 5" group at sea level, you can do it at 15000 feet....only the POI changes. Same with baro (to a much smaller degree).....you groups will be the same size, just your POI will change. Coriolis...spin drift...ect...all exterior forces. If you can shoot a 5" group shooting east, you can do it shooting west...only the POI changes.

    NOBODY said that exterior balistics have NO effect on the flight of a bullet...just no effect on accuracy...as long as the conditions holds through your string.
     
  10. Broz

    Broz Well-Known Member

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    One thing to maybe consider here would be the fact that air density/ altitude and air thickness comes into play with the needed twist rate for a given bullet. Some stability issues could come into play with altitude.

    Jeff
     
  11. 4xforfun

    4xforfun Well-Known Member

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    BROZ,



    Agreed....but....I would think that you would have to be on the very edge for that to happen. I have a 1-12 twist 30 cal that I planned on shooting the 187 BIB in 1K. That didn't work out, so I decided to try the new 200 Berger Hybrid, even though I was told it wouldn't stablize. VUNDERBULLET !!!...at least for now. Checking the stabilization program says that I am in the "yellow".... on the edge....something under 1.1 factor. I am not sure how it works, but if it works at sea level it should work at a higher alt...or is it visa versa? If it is visa versa, then yes....IF.... you are already on the edge. This IS something that I might worry about with this paticular bbl if I end up shooting at a match in Colorado. Right now I never see a range with an alt over 1500 feet.

    But this is really splitting hairs. What changes group size is harmonics....barrel time. Change the time the bullet is in the tube and you will change group size. You can change group size by changing neck tension...BUT once you find the right tension for a bbl, it never changes, regaurdless of powder types/charge/altitude/temp/ect..... Same with seating depth....you can change the shape/size of a group by changing seating depth of the bullet...but, once you find it, for that bullet, it won't change (in relationship to the lands) no matter what powder and speed you shoot.

    Interior balistics.
     
  12. BPreheim

    BPreheim Member

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    Any one who believes he is going to get the same group size when he does his development at 200ft 80 degrees and 80% humidity and then hunts at 9500ft 40 degrees and 15% humidity has not shot much in the west. how do you Keep your rifle warm on a ridge in a 20 MPH wind. rifle temp is just as important as ammo temp. Get the best groups you can at the conditions that as closely duplicate what your hunting conditions are going to be and accept that it may change.
     
  13. orkan

    orkan Well-Known Member

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    All of the academic stuff is cool and all. Still doesn't address the real world.

    Show me a 1/4 moa shooter in the field, at any elevation.
     
  14. 4xforfun

    4xforfun Well-Known Member

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    You are injecting a third parameter. Keeping the gun warm was not the issue...accuracy was. " WILL ACCURACY CHANGE WITH ELEVATION CHANGE" is the question. I do a bunch of my 1000 yard BR load work in the dead of winter. I keep the gun warm by shooting it...never let it get cold. In fact I get them HOT, just like at a match. After the first few shots I am ready to start shooting groups. Keeping the ammo warm is the most important factor. Mine sits in my warm truck untill it is time to shoot. Group size is no different at zero F than it is at 80F...as long as you keep things constant. I am just saying that all things (gun and ammo related) being equal.....temp, humidity, alt, baro(which takes the place of alt) ....EXTERIOR BALISTICS...will not change the group size. Only POI will change.

    Yes, you are all right ..to a degree....gun temps, ammo temps,...and on and on...all will change group size. You can't possably keep things warm in the field. That is a change in INTERIOR balistics. BUT..you are all thinking that the groups will get bigger. Why can't they get smaller? This is LRH...most everyone here shoots larger caliber magnums. I am pretty sure that everyone didn't do thier load development in1/10 of a gr increments and found that absolute PERFECT tune. Your ammo gets cold...you loose 10, 20, maybe 30 fps...who is to say that that decrease in velocity won't bring your gun MORE into tune? I had the oppisate happen to me at the 2011 IBS 1000 yard nationals in WV.....It was 92 -95 degrees. I have never shot in that warm of temp. the only way for me to shoot those conditions at home is to get things HOT and let the case cook in the chamber for xx seconds to get the ammo hot...or leave them in the sun..or the oven. Or load hotter ammo. Anyway..long story short, my gun shot better than it ever has in the hot weather. things got hot, velocity's went up ( a factor of interior balistics), the gun came MORE into tune, and I took home most of the wood.

    And as far as the shooter..you are absolutly right....higher elevation...less O2, breathing harder....mabe not as comforatable shooting on the range as he is at home....ect....can and will make groups get bigger.
     
    Last edited: Nov 17, 2012