Altitude vs. Barometric pressure

[lboom]

For every 1000 foot of elevation you go up you lose 1 inch of barometric pressure. When inputing data in programs the barometric pressure you get from local reporting is corrected to sea level.
Therefore the actual pressure at the altitude you are hunting at is barometric pressure minus (altitude divided by 1000). so an example if the local pressure is 30.00 and you are hunting at 7500 ft then the actual pressure is 30.00 minus 7.5 or 22.5

 

[Litehiker]

lboom,

That's "keerect" but if you are at 8,000 ft. and looking at a fog bank lowering itself on you you will have even more atmospheric density than calculated. That's where the weather station of the kestrel 4500 A/B comes into the calculation in accounting for relative humidity as well.

 

[lboom]

absolutly

 

[DocUSMCRetired]

Aerodynamics also come in to play if you are shooting against, cross, or with the wind, which further effect that influence.

Applied Ballistics Support Staff
[email protected]

 

[DocUSMCRetired]

Now if there were a way to use laser light to calculate average wind over the distance-to-target we'd have an even better solution. As it is the wind at the shooter's position is the most important reading in terms of effects on bullet wind drift and by taking two readings, the full wind and the wind on the line of firing to target the unit calculates a proper wind value.

QUESTION: With all other factors being equal will, for example, a .30 cal., 200 gr. bullet fired from a .300 Win mag cartridge with a 1:10 twist barrel drift more than the same bullet fired from a 1:8 twist barrel? I'd guess the faster twist rate gives more gyroscopic stability and perhaps less spin drift.

So to answer both of these paragraphs. The first one already exists, look up DARPA One-Shot. One of the engineers here at Appled Ballistics actually worked on it.

So your question. If you removed all other factors and looked only at the spin itself. The faster spinning bullet, would have more stability, resulting in more spin drift. Spin drift has a lot of factors associated with it, but the two main things are Time of Flight, and Stability. The bullet fired from the higher twist rate, will have more spin drift.

Applied Ballistics Support Staff
[email protected]

 

[BergerBoy]

So if I understand correctly, the most accurate way to use RSI is to leave the altitude field at 0' and just use the pressure field to imput the current pressure at my location no matter what the altitude.

I believe that pressure is absolute is the best way to set your ap/program to.

This is a GREAT topic.

Thank you Guys

 

[Litehiker]

Thanks Doc,

Now I have an answer to a question on spin drift versus twist rate that I've long wondered about.
So it's "More (higher) twist rate = more spin drift". Easy to remember.

But if more spin drift can be calculated then it's almost a moot point except that greater stability means less yaw at transonic speeds, no?

 

[1100 Remington Man]

How can this be right ? I thought 1 inch was a 1000' ft also. Should the pressure at 4000' be 25.92 ?
STANDARD CONDITIONS

ELEVATION STD. TEMP. F. STD PRESS. ABSOLUTE
0 59.0 29.92
500 57.2 29.45
1000 55.4 28.99
2000 51.9 28.07
2500 50.1 27.65
3000 48.3 27.21
3500 46.5 26.79
4000 44.7 26.37
4500 42.9 25.96
5000 41.2 25.54
5500 39.4 25.15
6000 37.6 24.76
6500 35.8 24.37
7000 34.0 23.99
7500 32.2 23.61
8000 30.5 23.24
8500 28.7 22.88
9000 26.9 22.52
9500 25.1 22.17
10000 23.3 21.82

 

[Mikecr]

The 1"/1Kft is a rule of thumb, and rules of thumb rarely represent truths.
Your chart there is also wrong.

Std conditions at 4,000 is 44.74 degF & 25.84 "Hg & 0% Rh
I throw in Rh because Std Conditions and Density Altitude are equal only at 0% Rh.

 

[1100 Remington Man]

The 1"/1Kft is a rule of thumb, and rules of thumb rarely represent truths.
Your chart there is also wrong.

Std conditions at 4,000 is 44.74 degF & 25.84 "Hg & 0% Rh
I throw in Rh because Std Conditions and Density Altitude are equal only at 0% Rh.

I copied and pasted from a link on this. I'm a pilot and the 1 inch per thousand feet is what we use.
Thanks

 

[Mikecr]

Makes no sense put your life on ridiculous rules of thumb..
Honestly, today I would think GPS would be the primary altitude source, or at least an altitude 'fix' (like position fix).

 

[1100 Remington Man]

Makes no sense put your life on ridiculous rules of thumb..
Honestly, today I would think GPS would be the primary altitude source, or at least an altitude 'fix' (like position fix).

Well this is what all planes use. Just FYI GPS is Accurate but not perfect either.

 

[Mikecr]

You cannot credibly compare atmospheric rules of thumb to GPS measurement(or any other actual measure)..

But this isn't about altitude anyway. It's about air density as seen by our bullets, and is only resolved through parameters we measure(Pressure, Temp, Rh).

 

[ChicSpandex]

An increase in altitude decreases the gravitational force. We have to worry about that when we are aiming at flea's eyes at 1000 meters and correcting for the gravitational attraction of the mountain to our side.