Author Topic: What is influence of altitude change on muzzle velocity?  (Read 393 times)

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Offline Clint007

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What is influence of altitude change on muzzle velocity?
« on: September 06, 2017, 01:56:14 PM »
Hi guys. found lots of online info about ballistics (mainly rifle) changes with altitude changes.

But...

I developed a match load for IDPA: 4.0 gr N320 under a 124 grain Precision Delta JHP.
This chronos out at about 1030-1040 fps (pf 128-130) at a temp in the 90s in my SP01 Shadow.  This is safely above the 125 pf requirement. I did this at 1000 ft (Phoenix).

I'm shooting at 5800 ft and the temps will be in the 70s.

I assume the muzzle velocity will be a tad slower due to temperature, but figured the air pressure (altitude) would have negligible effect on muzzle velocity, which is what will be chrono'd....

School me!

C
Huh?

Offline M1A4ME

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Re: What is influence of altitude change on muzzle velocity?
« Reply #1 on: September 06, 2017, 05:08:40 PM »
Can't say for a pistol load and pistol differences but higher altitudes means thinner air, means less air resistance/effect on the bullet which means the bullet maintains higher velocities farther from the muzzle which means less bullet drops at longer distances.

I wonder if there is a internet formula to calculate this for varying altitudes/air pressures/bullets?
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Offline Scarlett Pistol

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Re: What is influence of altitude change on muzzle velocity?
« Reply #2 on: September 06, 2017, 05:21:18 PM »
This might help a little. While you supply the muzzle velocity, you can set the distance increments to be small and see the impact at high and low elevations and the various temperatures.

https://www.federalpremium.com/ballistics_calculator/
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Online Joe L

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Re: What is influence of altitude change on muzzle velocity?
« Reply #3 on: September 06, 2017, 06:10:06 PM »
You guys are missing the question.  Altitude alone won't affect the bullet velocity measured a few feet from the pistol.  The powder temperature will.  The velocity a 100 yards downrange will certainly be higher at high altitude due to the lower air density but that isn't the question.  The IDPA velocity measurement for power factor determination isn't measured down range, it is measured near the muzzle.

Joe
CZ-75B 9mm and Kadet, 97B"E", two P-09's, P-07, P-10C

Offline Scarlett Pistol

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Re: What is influence of altitude change on muzzle velocity?
« Reply #4 on: September 06, 2017, 06:30:52 PM »
You guys are missing the question.  Altitude alone won't affect the bullet velocity measured a few feet from the pistol.  The powder temperature will.  The velocity a 100 yards downrange will certainly be higher at high altitude due to the lower air density but that isn't the question.  The IDPA velocity measurement for power factor determination isn't measured down range, it is measured near the muzzle.

Joe

Since the tool allows for temperature adjustments I was hoping it would help by setting the distance increments to just a few feet. Thus allowing the tool to mimic those close distances. If it isn't showing the real impact, I apologize.
"In God I trust. All others must supply data."
"I respect that the choice of pistols is very personal. Although, if someone is devoid at least one CZ, they've chosen the wrong pistols"  - Scarlett Pistol
1) CZ 75 Compact Classic - 9mm
2) CZ SP-01 - 9mm
3) "SP-01 Compact" (CZ 75 Compact)

Offline Clint007

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Re: What is influence of altitude change on muzzle velocity?
« Reply #5 on: September 06, 2017, 06:32:34 PM »
yep...I found all that interesting about long range ballistic effects, but for a match chrono....I assumed the altitude effect would be negligible.

Temperature, tho....yes. When I chrono'd these, it was averaging 1030-1040. I need to be at least 1008 fps.  So would 20-30 degrees make N320 burn different and sacrifice 20 fps?

C
« Last Edit: September 06, 2017, 06:36:27 PM by Clint007 »
Huh?

Offline painter

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Re: What is influence of altitude change on muzzle velocity?
« Reply #6 on: September 06, 2017, 07:06:23 PM »
Unless you're loading really short, I'd bump it to 4.1 and go shoot.
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Online Joe L

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Re: What is influence of altitude change on muzzle velocity?
« Reply #7 on: September 06, 2017, 08:03:28 PM »
Since the tool allows for temperature adjustments I was hoping it would help by setting the distance increments to just a few feet. Thus allowing the tool to mimic those close distances. If it isn't showing the real impact, I apologize.

No problem.  To use the calculator, you have to input the muzzle velocity, and that is what he is wanting to know at altitude compared to home.  At the same powder temperature, the muzzle velocity will be the same at altitude as it is at home, so the calculator can't help him with his question, that's all.   

It's a good calculator, however.   :) :)

Joe
CZ-75B 9mm and Kadet, 97B"E", two P-09's, P-07, P-10C

Offline 1SOW

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Re: What is influence of altitude change on muzzle velocity?
« Reply #8 on: September 06, 2017, 09:34:39 PM »
To learn from those who know:
Does air density in the case have any effect on muzzle velocity?
A pure guess for small cases would be...not significantly but mathematically yes.

Cartridge loaded at sea level vs cartridge loaded at 5k- 6k feet using the same load data?
« Last Edit: September 06, 2017, 09:37:39 PM by 1SOW »

Offline Clint007

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Re: What is influence of altitude change on muzzle velocity?
« Reply #9 on: September 06, 2017, 10:54:34 PM »
anyone know the relationship between velocity and temperature? Is it linear at these conditions?
Huh?

Offline jameslovesjammie

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Re: What is influence of altitude change on muzzle velocity?
« Reply #10 on: September 06, 2017, 11:31:01 PM »
anyone know the relationship between velocity and temperature? Is it linear at these conditions?

Some powders burn faster.  Some powders burn slower.  Some powders are relatively insensitive.  There is no single answer.

Offline tdogg

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Re: What is influence of altitude change on muzzle velocity?
« Reply #11 on: September 07, 2017, 12:17:22 AM »
Where are you going to be shooting?  Prescott action shooters?

If you really want to geek out and fine tune loads you need this:  http://www.neconos.com/details3.htm

Not really practical for pistol reloading but can be really helpful if you start wildcatting rifle cartridges.  I guess it could be useful to determine pistol max loads/pressures (if you trust a software to do that for you!).

Oh and gunpowder doesn't need oxygen for combustion, it is built in.  I can't imagine that the internal case volume pressure differences between sea level and 10k ft would be measurable/detectable over the standard reloading variability/noise.

Cheers,
Toby

Offline Clint007

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Re: What is influence of altitude change on muzzle velocity?
« Reply #12 on: September 07, 2017, 12:32:58 AM »
Farmington NM IDPA match

 OK I am going to make a load with a bit more juice and hopefully chrono both on site before the match!
Huh?

Offline IDescribe

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Re: What is influence of altitude change on muzzle velocity?
« Reply #13 on: September 07, 2017, 05:11:22 AM »
Some powders burn faster.  Some powders burn slower.  Some powders are relatively insensitive.  There is no single answer.

^^^^^^^^^^^ THIS ^^^^^^^^^^^



Some Googling suggests that N320 is somewhere between temp sensitive and relatively insensitive, but the difference between 90 and 70 isn't going to cost you much.  If you want to cover your bases, bump up a tenth of a grain or two.  Have you tuned that load for accuracy?  You might find a load with a tenth or two more performs better anyway.  Take the opportunity to do some load testing and see what the next couple rungs up the ladder do for you.  ;)



Also, the last time we had this discussion, we ended up with pictures of Finnish sauna girls on the first page of the thread, so I'd say you guys are slacking.  ;)

Offline Wobbly

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Re: What is influence of altitude change on muzzle velocity?
« Reply #14 on: September 07, 2017, 08:12:42 AM »
I have read somewhere that N320, among all its other endearing qualities, displays good temperature stability.

You have the chrono. Leave some test ammo at 90F, and cool some other test ammo to 70F.  Should be a quick backyard test.
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