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Power Factor At Higher Elevation/altitude


alellis

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Not sure if its elevation or altitude I am guessing its elevation if your feet are on the ground and altitude if you are in the air. Someone no doubt will tell me.

Anyways what I want to know is if I go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect my load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

al

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Not sure if its elevation or altitude I am guessing its elevation if your feet are on the ground and altitude if you are in the air. Someone no doubt will tell me.

Anyways what I want to know is if I go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect my load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

al

i'm no physics major, but everytime i chrono at a match in reno, which is over 4K feet, the velocity is greater-sometimes over 35 fps. and my home range is basically at sea level.

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At this yeard Colorado Sectional I had the same PF/FPS as I did at home.

About a 14,000' difference in altitude.

YMMV

FM

You should see an increase of one magnitude or another, but it should be a very small difference, perhaps not noticeable. If the ammunition was loaded at the same altitude, then the only factor is the resistance of air in the barrel, and the resistence of air from the barrel to the chrono. I can't see it as being significant. Out at 100yds it would probably have a considerable impact, especially in the rifle sports.

H.

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What Houngan says-- the bullet only has to bash through 10-15 feet of air before it gets to (hopefully not "hits") the chrono. A typical bullet only loses about 3 to 5 fps doing that at sea level, so the altitude change will be minimal. At longer ranges (read: rifle) it can be important.

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Anyways what I want to know is if I go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect my load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

al

What all you say about not noticeable difference. In theory, as altitude increases, air density, pressure, and temperature decrease, so less drag more bullet velocity, but the temperature(lower) of the propellant (powder) in a cartridge at the instant of ignition can have an effect on chamber pressure. If a gun is sighted in on a target range at some temperature, and then is fired in another environment in which the propellant temperature is different, the muzzle velocity of a bullet can be different compared to what it was when the gun was sighted in. The difference in muzzle velocity because powder temperature. The temperature sensitivity of different powders varies from one type to another. Only the manufacturer of any type of powder can quantify the temperature sensitivity of that powder to a handloader.

It´s not easy to find a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

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Our squad debated this at the Cheyenne Shootout.

I chronoed at 925-930 FPS here at Sea Level, but

Chronoed at 940-948 FPS in Cheyenne.

seemed most shooters who had a known load at lower elevations found 10-15 FPS gain by adding 5000 feet of elevation.

Bottom Line.

Not enough to talk about.

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Anyways what I want to know is if I go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect my load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

al

What all you say about not noticeable difference. In theory, as altitude increases, air density, pressure, and temperature decrease, so less drag more bullet velocity, but the temperature(lower) of the propellant (powder) in a cartridge at the instant of ignition can have an effect on chamber pressure. If a gun is sighted in on a target range at some temperature, and then is fired in another environment in which the propellant temperature is different, the muzzle velocity of a bullet can be different compared to what it was when the gun was sighted in. The difference in muzzle velocity because powder temperature. The temperature sensitivity of different powders varies from one type to another. Only the manufacturer of any type of powder can quantify the temperature sensitivity of that powder to a handloader.

It´s not easy to find a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

Whoa whoa whoa, nobody said anything about temperature differences, just alititude. While it is colder on average, or if we're talking about a perfect laboratory experiment with the same chunk of matter at two different pressures, then PV=NrT will dictate a lower temperature which has a much bigger effect than lower air pressure, as you've pointed out.

But, as presented he was only asking about elevation, which inherently changes only pressure. Temperature is still a product of the local weather and can't be predicted solely by altitude.

H.

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But, as presented he was only asking about elevation, which inherently changes only pressure. Temperature is still a product of the local weather and can't be predicted solely by altitude.

H.

This is not completely accurate. When I was skydiving, the rule of thumb was that there was a 3 degree change in temperature per 1000 feet. So a 15,000' jump over Amelia Island, FL was a good way to cool off in the summer. Your goggles would usually fog up on the way down, which is why most Florida jumpers don't bother.

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But, as presented he was only asking about elevation, which inherently changes only pressure. Temperature is still a product of the local weather and can't be predicted solely by altitude.

H.

This is not completely accurate. When I was skydiving, the rule of thumb was that there was a 3 degree change in temperature per 1000 feet. So a 15,000' jump over Amelia Island, FL was a good way to cool off in the summer. Your goggles would usually fog up on the way down, which is why most Florida jumpers don't bother.

As I said, it's colder on average, but we're talking about two distinct occurrences here, the first match and the second match. There is no information whether the first was in February at sea level or the second at the start of August. The only information we were provided was that there was an altitude change, which only inherently means a pressure change. It suggests a temperature change.

H.

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As I said, it's colder on average, but we're talking about two distinct occurrences here, the first match and the second match. There is no information whether the first was in February at sea level or the second at the start of August. The only information we were provided was that there was an altitude change, which only inherently means a pressure change. It suggests a temperature change.

H.

Houngan:

As I said:

As altitude increases, air density, pressure, and temperature decrease, so less drag more bullet velocity. I was talking about the characteristics of the standard atmosphere while altitude increase. You can see, it is not only a pressure change.

Alellis wanted to know is if he go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect his load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

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As a Colorado shooter who typically chronos at 8000 ft. (Clear Creek), I'm only worried when I go down to sea level, e.g., Nationals in Barry. The 500 ft. increase in altitude should be negligible and is actually going in the "better direction" per all the reasons stated above.

Greg Lent (Chronoman) wrote an article for Front Sight a couple of issues ago where he shares his (vast) experiences with the topic. I believe it's more of a concern for high altitude, home range types going down significantly in elevation vs. the other way. Powder sensitivity to temp. and position in the case is a variable that I believe Greg covered in the article as well.

My free advice: why cut it so close to where any of these variables make a difference? I've been close to going minor at major matches early on where I never want to go there again! It's not a pleasant experience. PFs of 170-172 seem to give me the cushion that I want in this sport (for major of course).

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As I said, it's colder on average, but we're talking about two distinct occurrences here, the first match and the second match. There is no information whether the first was in February at sea level or the second at the start of August. The only information we were provided was that there was an altitude change, which only inherently means a pressure change. It suggests a temperature change.

H.

Houngan:

As I said:

As altitude increases, air density, pressure, and temperature decrease, so less drag more bullet velocity. I was talking about the characteristics of the standard atmosphere while altitude increase. You can see, it is not only a pressure change.

Alellis wanted to know is if he go to shoot at a different range and it is 500 ft higher ( above sea level) how will it affect his load and is there a formula to figure out how much to increase/decrease the powder charge to maintain power factor.

I'm not disagreeing with you except that temperature, in his question, was not addressed. The only thing you can COUNT on at higher elevations are lower density of air and lower pressure, not necessarily lower temperature. While on average it's always going to be cooler the higher you go, it gets below zero here in Kentucky, it gets above 70 in the Rockies. Also, adjusting for temperature is its own thing, and affects the performance the same regardless of altitude. So, in real life you are giving him more useful advice, but I was trying to point out that in his question/thought experiment, the only reliably different variable, air pressure/density would not affect his velocity very much.

H.

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At this yeard Colorado Sectional I had the same PF/FPS as I did at home.

About a 14,000' difference in altitude.

YMMV

FM

You should see an increase of one magnitude or another, but it should be a very small difference, perhaps not noticeable. If the ammunition was loaded at the same altitude, then the only factor is the resistance of air in the barrel, and the resistence of air from the barrel to the chrono. I can't see it as being significant. Out at 100yds it would probably have a considerable impact, especially in the rifle sports.

H.

What Houngan says-- the bullet only has to bash through 10-15 feet of air before it gets to (hopefully not "hits") the chrono. A typical bullet only loses about 3 to 5 fps doing that at sea level, so the altitude change will be minimal. At longer ranges (read: rifle) it can be important.

Maybe the temperature differnce (70 versus 102) made up for the alltitude difference resulting in the same chrono results?

This was a pistol only match and I know rifle would be a whole other can of worms.

It was just my perception that with my 38 Super Comp loads that altitude didn't make any difference.

FM

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At this yeard Colorado Sectional I had the same PF/FPS as I did at home.

About a 14,000' difference in altitude.

YMMV

FM

You should see an increase of one magnitude or another, but it should be a very small difference, perhaps not noticeable. If the ammunition was loaded at the same altitude, then the only factor is the resistance of air in the barrel, and the resistence of air from the barrel to the chrono. I can't see it as being significant. Out at 100yds it would probably have a considerable impact, especially in the rifle sports.

H.

What Houngan says-- the bullet only has to bash through 10-15 feet of air before it gets to (hopefully not "hits") the chrono. A typical bullet only loses about 3 to 5 fps doing that at sea level, so the altitude change will be minimal. At longer ranges (read: rifle) it can be important.

Maybe the temperature differnce (70 versus 102) made up for the alltitude difference resulting in the same chrono results?

This was a pistol only match and I know rifle would be a whole other can of worms.

It was just my perception that with my 38 Super Comp loads that altitude didn't make any difference.

FM

Can't say for sure, but as we've been yapping about, the temperature difference is going to have a larger effect, so it could very well have evened-out the altitude difference.

H.

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Effect temperature has to do more with the bullet thrust (force completely expended in the first instance, which moves the bullet through the barrel then air. Powder temp. sensitivity). Now, exterior ballistic, in theory, aerodynamic drag is directly related to the air density flowing past the bullet. As altitude increases, air density decrease, so less drag more bullet velocity, assuming same inicial thrust.

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