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Muzzle Velocity at Different Altitudes

 
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  #1  
Old 05-07-2009, 06:30 PM
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Muzzle Velocity at Different Altitudes

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

I live at 4550 ft above sea level. Itís 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. Iím 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 ftÖ right?

Are changes in muzzle velocity caused mostly by the powders sensitivity to temperature changes?
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Old 05-07-2009, 08:36 PM
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Re: Muzzle Velocity at Different Altitudes

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
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Old 05-08-2009, 09:45 AM
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Re: Muzzle Velocity at Different Altitudes

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.
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Old 05-08-2009, 10:54 AM
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Re: Muzzle Velocity at Different Altitudes

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.
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Old 05-08-2009, 01:03 PM
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Re: Muzzle Velocity at Different Altitudes

Quote:
Originally Posted by Michael Eichele View Post
albiet many of us change our MV based on temprature and powders burn rates in a given temp.
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.
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Old 05-08-2009, 01:25 PM
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Re: Muzzle Velocity at Different Altitudes

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
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Old 05-08-2009, 01:34 PM
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Re: Muzzle Velocity at Different Altitudes

Quote:
Originally Posted by bsl135 View Post
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

+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
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