What impact does altitude have on bullet flight?

[bobinpa]

I would love to learn about this. I want to understand why altitude effects bullet flight..... Can anyone tell me? Is there a web site out there that explains this?

 

[Forum Boss]

Basic idea is that higher altitude = thinner air = less aerodynamic drag, so bullet doesn't slow down as quickly.

Of couse, there are other factors you may commonly encounter at higher altitude ranges, e.g. different humidity, different average temperatures etc.

 

[memilanuk]

In basic terms, as your altitude increases, the density of the air the bullet must travel thru decreases, thereby reducing the drag on the bullet. Generally, the higher the altitude, the less the bullet will drop.

Example: I shoot at a couple ranges here in the Pacific Northwest. Both are at 1000' ASL or less. Takes about... 29-30moa or so to get from 100yds to 1000yds with a Berger 155gr VLD @ 2960fps. Go to someplace with a much higher altitude, like Raton NM at 6600' ASL, and it takes about 24-25moa to do the same.

One thing to remember, it's not so much the altitude, as the barometric pressure. 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.

Might try reading thru here if you have more ballistics questions:

http://exteriorballistics.com/ebexplained/index.cfm

 

[rayjay]

I once read an interesting story on a Korean war ace. It was being told by someone that flew on his wing for awhile. One of the first times they flew together they got into a long tail chase with a Mig and the ace started firing from "well out of range". Next thing you know he had gotten some strikes and the Mig started slowing down and they closed in for the kill.

The ace had done some study on the effects of extreme high altitude [ 30,000 to 40,000'] on the trajectory of the 50 cal and knew how much to hold off for those effects. This allowed him to make kills on Migs that other flyers would not have been able to get hits on.

 

[butchlambert]

I think that density altitude affects it most.
Butch

 

[mikecr]

Density altitude.. Pressure altitude..
Best to FORGET ALTITUDE

Catshooter described the conditions of concern.
Barometric pressure,absolute/station/local), temperature, and humidity.

 

[butchlambert]

You will start seeing a lot of shooters starting to carry their DA meters to the matches this year and more in the following years.
Butch

 

[Rustystud]

Sorta Unrelated but interesting about high flying aircraft and firing 50 cals or 20mms. The F-111s during the Vietnam war were armed with 50s and 20mm. They could fly faster than the bullets could maintain superior velocity. In cases where the trailing pilot fired a string of fire above the intended target both the target and shooter could become vulnerable to running into their on fire if they remained on a straight course. In either level flight or decending flight.
Rustystud

 

[mikecr]

You will start seeing a lot of shooters starting to carry their DA meters to the matches this year and more in the following years.
Butch

I know, we'll all be twisting knobs on the end of our barrels..

But technically, neither Pressure altitude or density altitude are measured by any meter. These terms are calculated, and cannot be measured. They aren't real.

Real altitude,that questioned) can be measured with GPS, but as Catshooter implied, it has zero meaning in ballistics,Internal or external).

The moderator really summed it up well, and others have tried to clear a path through confusion common with altitude.

I think that density altitude could be useful in ballistics. But there seems nothing in shooting that can be rolled up into a single package and pass all tests.
So in my lifetime, I doubt I'll see anything but confusion and misapplication regarding altitude.

 

[memilanuk]

I've seen density altitude,DA) discussed on other forums... either in a tactical scenario with nomographs on laminated cards, or for rimfire benchrest. If I recall, Mike Ratigan makes a brief reference to DA in his book as well. I know enough about it,DA) to know I don't know enough about it to really say anything more about it, other than some people who are far more 'into it' than I seem to think it is significant. Then again, I don't generally tune my loads at the range... the nature of the games I play in don't lend themselves to that style of loading so DA may be a non-issue for me.

So far a Kestrel 4000,which does output DA if you are so inclined) set to display temperature, local,station) pressure, and time plus a PDA running Exbal has been a pretty good solution for me. My Palm Tungsten E has been acting up and I'm about due for a 'new' PDA... probably a used PocketPC off of eBay.

 

[rstreich]

...Barometric pressure,absolute/station/local), temperature, and humidity.

Hmmm, exactly the variables that a Kestrel uses to derive the density altitude. Isn't that curious?

But technically, neither Pressure altitude or density altitude are measured by any meter. These terms are calculated, and cannot be measured. They aren't real.

That's not quite true. These values are derived from measured values. True, they're calculated, but since the results are deterministic, there just as real as the measured values.