Muzzle Velocity at Different Altitudes

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by One_Duck_Limit, May 7, 2009.

  1. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    I’ve been reading all I can find about the affects of altitude and pressure on ballistics. I’ve got a hypothetical question.

    I live at 4550 ft above sea level. It’s 60 degrees and sunny at my local range, my average chronograph reading is 3050 fps.

    I jump in my truck and head to my favorite spot to practice a few long range shots. I’m now 4000 ft higher at 8550 ft above sea level. The temp is still 60 degrees and sunny.

    Will my average speed still be 3050 fps? Does the lower barometric pressure have such an affect that my average speed will increase?

    I understand the bullet will drop less because of the pressure decrease.

    Given the same temperature, the muzzle velocity at sea level would be the same as 10,000 ft… right?

    Are changes in muzzle velocity caused mostly by the powders sensitivity to temperature changes?
     
  2. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    I believe you'll see the same muzzle velocity at any altitude.

    As you pointed out, the difference is how well the bullet retains it's velocity downrange. Higher altitudes = less dense air and better velocity retention but there's nothing about the barometric pressure that should affect muzzle velocity.

    -Bryan
     

  3. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Thanks Bryan.

    So I need to develop a load with a nice low ES using a temp sensative powder and go from there. Take an average and use that number in my program to produce a drop chart. Muzzle velocity is what it is and I don't need to worry about it as I create and tweak my charts.
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    Theoretically, your MV could change an ever so slight amount due to less air pressure as the bullet moves down the bore, however, these changes would be so minimal that they wouldnt bear any real merit.

    I nor anybody I know corrects MV for a given air density albiet many of us change our MV based on temprature and powders burn rates in a given temp.
     
  5. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Is this something I can calculate? Or is it more shoot through the chrony during different temps?

    I shoot a 7mm Rem Mag. I'm in the process of working up a load using 168g Berger VLD's using Retumbo and Fed. 215's or 210's.
     
  6. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Hodgdon's powders are well known for being insensitive to temperature. I'd be surprised if you could measure a significant difference in average velocity over a 50-60 degree temperature difference.

    Regarding primers;
    Back when I shot 7mm Rem Mag, I struggled a lot with MV variation until I tried standard (non-magnum) primers. As I recall, the average MV wasn't much different, and the variation went way down. I've heard others observe this too.

    -Bryan
     
  7. AJ Peacock

    AJ Peacock Well-Known Member

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    +1 re: the non mag primers in the 7mm RM. I've experimented for 30+ years with the 7RM and have always had the best luck with non mag primers. My current powder is RL-22 and Rem 9 1/2 (non mag) primers, norma brass and 160AB. I'm getting sub .2moa groups from a 24" sporter weight barrel!

    AJ
     
  8. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Thanks for the primer info. I've got a few CCI BR2's and Federal 210M's. I'll work up a load using one of those.

    I've got another question about powder. My 7mm Rem Mag has a 26" barrel. My buddy's 7mm Rem Mag has a 24" barrel. To get the best performance would the 24" barrel require a faster burning powder than the 26" barrel? Bullet will be the same, Berger 168 VLD.
     
  9. AJ Peacock

    AJ Peacock Well-Known Member

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    Not enough difference to matter. I'd say that in all my internet reading, the current best powder for the 7RM is RL-22. I had good loads with IMR-4350, IMR-4831 and H-4831. After reading about RL-22, I bought a pound in Sept 2007, and haven't touched any other powder since that date for my 7RM's. I bought 2x8lb jugs of the powder a couple weeks later.

    Good luck and keep us informed.

    AJ
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    It is not something you can calculate. You will need to test some at 0 degrees and 80 degrees or at the low end of the temps you hunt in and the high end. If the difference is only between 30-60 degrees, using extreme powder, it will be of little concern. But if you go from 10 degrees one hunt and 90+ the next, you will want to test your load at each temp. Then you can calculate the difference per degree for an average loss or gain.

    Many many factors go into what makes a load sensitive or not sensitive to temp changes and those variables are not always predictable.