How Altitude and Air Pressure Influence Ballistics
One of our readers asked “What effect does altitude have on the flight of a bullet?” The simplistic answer is that, at higher altitudes, the air is thinner (lower density), so there is less drag on the bullet. This means that the amount of bullet drop is less at any given flight distance from the muzzle. Since the force of gravity is essentially constant on the earth’s surface (for practical purposes), the bullet’s downward acceleration doesn’t change, but a bullet launched at a higher altitude is able to fly slightly farther (in the thinner air) for every increment of downward movement. Effectively, the bullet behaves as if it has a higher ballistic coefficient.
Forum member Milanuk explains that the key factor is not altitude, but rather air pressure. Milanuk writes:
“In basic terms, as your altitude increases, the density of the air the bullet must travel through decreases, thereby reducing the drag on the bullet. Generally, the higher the altitude, the less the bullet will drop. For example, I shoot at a couple ranges here in the Pacific Northwest. Both are at 1000′ ASL or less. I’ll need about 29-30 MOA to get from 100 yard to 1000 yards with a Berger 155gr VLD @ 2960fps. By contrast, in Raton, NM, located at 6600′ ASL, I’ll only need about 24-25 MOA to do the same. That’s a significant difference.
Note that it is the barometric pressure that really matters, not simply the nominal altitude. The barometric pressure will indicate the reduced pressure from a higher altitude, but it will also show you the pressure changes as a front moves in, etc. which can play havoc w/ your calculated come-ups. Most altimeters are simply barometers that read in feet instead of inches of mercury.”
As Milanuk states, it is NOT altitude per se, but the LOCAL barometric pressure (sometimes called “station pressure”) that is key. The two atmospheric conditions that most effect bullet flight are air temperature, and barometric pressure. Normally, humidity has a negligible effect.
It’s important to remember that the barometric pressure reported on the radio (or internet) may be stated as a sea level equivalency. So in Denver (at 6,000 feet amsl), if the local pressure is 24″, the radio will report the barometric pressure to be 30″. If you do high altitude shooting at long range, bring along a Kestral, or remember to mentally correct the radio station’s pressure, by 1″ per 1,000 feet.”
You can do your own experimental calculations using JBM Online Ballistics (free to use). Here is an extreme example, with two printouts (generated with Point Blank software), one showing bullet trajectory at sea level (0′ altitude) and one at 20,000 feet. For demonstration sake, we assigned a low 0.2 BC to the bullet, with a velocity of 3000 fps.
Trajectory of Bullet fired at Sea Level
Trajectory of Bullet fired at 20,000 feet
if you want to learn more about all aspects of External Ballistics, ExteriorBallistics.com provides a variety of useful resources. In particular, on that site, Section 3.1 of the Sierra Manual is reprinted, covering Effects of Altitude and Atmospheric Pressure on bullet flight.
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Tags: Air Pressure, ballistics, Barometer, Density Altitude, JBM Ballistics, Milanuk, Point-Blank
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
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?
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.
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).
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 …