Comments on: How Altitude and Air Pressure Influence Ballistics

By: Steve Hurt

Steve Hurt — Mon, 28 Jan 2013 23:55:43 +0000

Actual temperature and station pressure are the left and right leg in this long walk.
They don’t just affect BC and therefore MOA calculations.
They directly influence load pressures and therefore velocity, as most serious shooters well know; but less well understood are their influence on projectile stability.
All good PDA or equivalent portable ballistic programs will factor in these parameters into velocity and trajectory calculations. Few, if any, however include gyroscopic stability (Sg)requirements for specific ranges or conditions.
In my experience, the guiding principle of Sg 1.4 is OK, but the tolerance range does vary depending on distance and conditions.
For instance, Sg’s of 1.4 – 2.0 seem to work reliably well at ranges out to 500 yards, but Sg’s in the 1.3 – 1.6 band appear better suited to distances beyond 500. (The slightly lower Sg requirement for longer range allows the projectile to nose over to Point on Flight, rather than an aircraft landing approach)
Since the Sg variable requirements are not normally included in these programs, they need to be considered and evaluated before the trip, using the the Miller stability program available on this website.
It’s fine to have the theory of external flight calculations at your finger tips, but all that flies out the window if the bullet loses stability on the way.
Actual air pressures and temperatures are far more important than many make allowance for …

By: MikeCr

MikeCr — Tue, 14 Sep 2010 00:32:26 +0000

I calibrate my Kestrel ALT at actual sea level here on the beach. But you could with GPS.
I only use station pressure, Temp, RH.
No altitude or density altitude or pressure altitude(nothing calculated).

By: Jon

Jon — Sat, 11 Sep 2010 22:37:56 +0000

If you have access to a portable ballistics calculator, e.g. Ballistic on the iPhone/Touch or Shooter for Android, you can enter values in the field. In which case it makes sense to have some sort of device with a barometric pressure sensor. A Kestrel or watch is one way to do it. Another is a GPS. If you have station pressure (sometimes referred to as “Ambient Pressure”) you do not need to enter altitude.

If you do not have access to station pressure, it’s better than nothing to use both corrected pressure (from the nearest airport, typically) and your current altitude, e.g. airport @ 4800 ft. reports 29.86 corrected pressure. Current altitude is 6400 ft. Just leave the pressure at that, indicate to the program that it is corrected pressure, and enter an altitude of 6400 ft.

By: Pao

Pao — Mon, 17 Aug 2009 18:49:23 +0000

Terminology sometime gets in the way of the shooter.

I’m always confused with the term barometric, station, atmospheric, or absolute pressure. Howerver, this is what I’ve always use. I always interpret BAROMETRIC PRESSURE to be the corrected pressure as reported by the local weather station, airport, and as displayed by Kestrel meter. I interpret STATION PRESSURE to be the actual or absolute pressure the earth’s atmosphere is asserting on a specific location, and as displayed by KESTREL by setting reference ALTITUDE to “0ft”. I did this because in my mind, I’m thinking the designer of any ballistic program would use STATION PRESSURE in its math. Therefore, STATION PRESSURE is what I use as entry to the PRESSURE box of the program, in my case EXBAL. I don’t use the calculated pressure generated by the software because, like the guys say in the link above, a weather front will effect the STATION PRESSURE that is not seen by the Calculator. Even though the Kestrel meter will display Barometric Pressure (corrected pressure to sea level), it may not be accurate unless you have set the reference altitude to the exact elevation (either from topo map or a landmark). Thus, this option is not very convenient. Also, Kestrel will display ALTITUDE given a reference BAROMETRIC PRESSURE, but again, we don’t always have access to obtaining this reference pressure when we’re up in the mountain somewhere. Thus, ALTITUDE as displayed by Kestrel will not always be accurate in my mind. So, if ALTITUDE is not accurate, neither will the calculated STATION PRESSURE since it is based on it. In EXBAL, I always set my Altitude to zero and manually enter the Station Pressure as displayed by Kestrel. I do this during sigh-in and during my hunt.

My question to you shooters;

Are you guys doing the same thing I’m doing?

Do you use the calculated pressure instead?

If you allow the program use the calculated pressure, what/where/who is your most convenient and reliable source of reference altitude?

By: Deepak Saxena

Deepak Saxena — Sun, 02 Nov 2008 11:14:20 +0000

Dear Sir,

Somebody told me when a bullet hits, the pressure at which it pierce the human body is around 2 Bar? Is that correct?

Regards,

Deepak