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Altitude & Temperature Effects on a Bullet?

 
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  #8  
Old 03-04-2005, 01:22 PM
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Re: Altitude & Temperature Effects on a Bullet?

Altitude is of little value without measuring actual air pressure. At my location, normal variation in pressure is 930-1070 hPa.

For a bullet of about Bc450 v2830fps this would translate to about 1/12 MOA per 10 hPa at 550yds.
At shorter ranges don't bother as temperature often cancels the effect of air pressure.

As to air temperature, same bullet will lose about 1/7 MOA at 550yds per 5 degrees Celsius from standard metro atmosphere.

Using slow burning powder such as Norma MRP or VitN160 with CCI primers adding +1/3 MOA at 550yds is sufficient all the way down to freezing temperature on ammunition.

So really nothing to bother about at shorter ranges.
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  #9  
Old 03-04-2005, 09:58 PM
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Re: Altitude & Temperature Effects on a Bullet?

[ QUOTE ]
Altitude is of little value without measuring actual air pressure.

[/ QUOTE ]

Sierra22, actually altitude has no value, at least for me, the way I run the #'s through Exbal. For me I had used them as a guideline to help me get close to a pressure close to his. For example we hunt in Maine at an alt of 4328' and took the barometric pressures we had for there and plugged them in for his 4500'

Now, although this is not exact, this will put me in the ball game at least for the illustration purposes needed for his post. I nolonger use altitude in any of my calculations only the barometric pressure, as not only does this change for elevation it also changes with the weather, so this is the best method to use to get the most accurate ballistic data.

Here in NY at 830' above sea level during the month of Feb my Kestrel 4000 said the average barometric pressure was 29.43inHg with a high of 30.34inHg and a low of 28.43inHg but this changes with the seasons and as the weather changes.

So one can see that even though I had not moved, the pressure had a pretty good swing and if I had sighted in while the pressure was 28.43inHg and then took shots at game while the barometric pressure was at 30.34inHg without running the numbers again I would be 2.4" off at 500yrds and 22.9" at 1000yrds even though nothing else had changed.

Thanks,
Ben
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  #10  
Old 03-05-2005, 03:59 AM
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Join Date: Jun 2001
Location: Palmer, Alaska
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Re: Altitude & Temperature Effects on a Bullet?

"Barometric" pressure is useless "unless" you know the altitude "you" are at, as it is station pressure (raw pressure) corrected to sea level.

If you are at 4500 ft ASL, setting the Kestrel 4000's reference altitude to zero on the BP screen will produce a station pressure reading quite a lot lower than the actual barametric pressure because there is less pressure at 4500'. If you set the reference altitude on the BP screen to 4500 ft, it will simply raise the pressure reading and correct it to what pressure would be at sea level directly beneath us... not exactly the station pressure number at our location though, and that's what is important. At that point, you'd have to enter the barometric pressure into exbal, but also tell it what altitude you are at so it could convert the BP back to your actual station pressure in order to calculate trajectory correctly. Hope that helps some.
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  #11  
Old 03-05-2005, 10:43 AM
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Re: Altitude & Temperature Effects on a Bullet?

Brent,
Altitude should only be used when you have no means of utilizing the Barometric pressure.

I use uncorrected for altitude Barometric Pressure.

Uncorrected for altitude barometric pressure simply means that I set the Kestrel to a constant elevation of zero feet. This is because you want a true barometric pressure reading. 28.27 bar at sea level is the same as 28.27 bar at 1,000 ft & 5,000ft & 10,000ft.

I use to use the alt in my equations but after speaking to Ward at Snipertools he said I was doing it all wrong an that you infact want to use uncorrected for altitude barometric pressure and when inputting the info into my palm or laptop to leave alt at zero and input the uncorrected pressure to get the most accurate readings. Just so you know this the way he teaches his students in his Precision Shooting 1 Class.

I'm sure if you need further explanation why he does this he could explain it in much more detail as when he told me what to do, I did not question it and just did it as he has a whole lot more experience with this than I.

Thanks,
Ben
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  #12  
Old 03-05-2005, 02:30 PM
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Join Date: Jun 2001
Location: Palmer, Alaska
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Re: Altitude & Temperature Effects on a Bullet?

Shadowman,

Didn't mean to confuse, and should have stated that sea level, or reported pressure is generally refered to as barometric pressure, where pressure at altitude is generally refered to as station pressure. What we see on the weather reports, reguardless of the cities altitude, is the barometric pressure corrected to mean sea level. So if you head to the hills in Colorado and the local weather man on the radio reports 30.00" hg, you'd better know your altitude in order to calculate correct trajectories for that day, because that BP was corrected to sea level, not the actual pressure at your elevation.

Using the Kestrel 4000 or other barometer which has been calibrated will give you the actual BP at your location (station pressure), but if you use a ballistic program and enter your elevation as "anything" but zero when doing so, the program will correct it yet once again and calculations will be wrong.

There is nothing wrong with using altitude in your calculations, as long as the BP you also enter was in fact corrected to sea level pressure. The program will then calculate the station pressure for you.

The nice thing about using a barometer, like the Kestrel, in the field is, you always have the actual pressure at your fingertips.
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Brent Moffitt
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  #13  
Old 03-05-2005, 09:30 PM
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Join Date: Dec 2004
Location: BINGHAMTON, NY
Posts: 166
Re: Altitude & Temperature Effects on a Bullet?

barometer, barometric pressure, station pressure - see that when I look at those words I see one thing and when you look at them you see three. Sorry if I had used the wrong words and got you confused. [img]/ubbthreads/images/graemlins/blush.gif[/img]

Thanks,
Ben
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  #14  
Old 03-06-2005, 12:32 AM
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Location: NC, oceanfront
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Re: Altitude & Temperature Effects on a Bullet?

I'm with you shadowman, just use raw pressure. Always.
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