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density altitude

How does density altitude affect a bullet in flight?

I shot today, 634 yards, shooting 260AI, 123gr lapua, 3104 fps. Strelok showed 10.1moa hold, I was holding 9. Correct fps from strelok showed 3242 fps for a 9 moa hold. I done some math, DA was 5454, plugged that number in, strelok showed 9.1 moa.

How does DA affect the bullet? Thinner air less drag? Thicker air more drag? Do I need to always enter DA for the day I'm shooting? Guessing so, but would like your opinion...
 
You don't give your sight in altitude nor the altitude where you were shooting, but the higher the altitude the thinner the air. Thinner air equals less drag. It is important to put your altitude in the program. There is a big difference between sea level and 5,000 ft above sea level.
 
Here is something else I noticed that I believe is hooked into altitude, humidity air density etc.. Last year I moved FROM Houston, Texas to Chandler, Arizona. Houston is about 25f.t. above sea level and Arizona, where we shoot is in the mountains. There is almost no humidity in Arizona and horrifying humidity in Houston. I have 5 F-Open rifles and I had all of them shooting tiny dots at 300 and they shot very well out at 600 / 1000 IN HOUSTON.. When I started shooting out here, my scores were completely in the toilet! So I loaded up all of them and went to the range to shoot them at 300yards.. My .260A.I. actually shot better than in Houston, however, the others all shot like a blunderbuss! I had to COMPLETELY redo the loads. It took awhile to figure out the loads and once I got them shooting, they shot great. Now that was going from Houston to Arizona.
Last Spring I went back to Houston for the TSRA Long Range State Championship. I had a pleasant surprise>>> my rifle shot just as well in Houston as the load I developed in Arizona! Going from Houston TO Arizona wreaked havoc with my loads>>>vice versa was unchanged!
 
Based on gps, shooting at 1725 ft, sight in at 1695, 79 degrees, 54%humidity, pressure at local airport said 30.16, phone said 28 something, due point 54 degree, high pressure in the area today. 2mph wind at 6 clock.

Sight in at 200 yards.

So it changes daily, I need to figure it daily and plug in DA I presume. Didn't realize it made that much difference. Didn't affect my 280AI that much, but the bc on the bullet is higher. That would explain what I saw.
 
Shootdots,

So why do you think the load changed from TX to AR and stayed the same when back in TX?

I'm guessing that my crono was correct at 3104(average) but due to DA the BC of the bullet was different? So how do the bullet makers come up with a BC of a bullet? Do they shoot them in controlled lab? Or outside where everything changes? Maybe I'm over thinking this and just need to figure DA for everytime I go shooting, but would like to understand how this works.
 
One big thing to remember is don't get station pressure and barometric pressure confused totally different barometric is corrected for sea level most shooting apps prefer station pressure set humidity at 50% and forget about it DA changes constantly ( within reason of course)
 
in a nutshell density altitude is pressure altitude corrected for non-standard temperature basically its effected by hot or cold
 
Shootdots,

So why do you think the load changed from TX to AR and stayed the same when back in TX?

I'm guessing that my crono was correct at 3104(average) but due to DA the BC of the bullet was different? So how do the bullet makers come up with a BC of a bullet? Do they shoot them in controlled lab? Or outside where everything changes? Maybe I'm over thinking this and just need to figure DA for everytime I go shooting, but would like to understand how this works.
I do not know the "whys" of that phenomenon.. To be honest, I simply develop a load, chrono it and make the necessary come-ups for whatever distance I will be shooting at. 99.9 pct. of the time the come-ups are close enough that usually I am 1/2 minute or less of "correction" from the initial come-up. I don't worry as long as I can find the X-Ring at 600 and closer in two sighters or less, maybe 3 or 4 tops at 1000..
 
So to any pilots out there... How does DA affect flight? Thinker air requires more power to stay at same altitude, less DA requires less power?

Same as bullet in flight? Maybe thinner air reduces drag on air craft, thinker air makes more drag? If so...I think I'm correct on this... How do bullet markers come up with a BC?

Thinking about all that affects drag(drag=BC) Then BC has to be done in a controlled environment. If not then published BC is (maybe) not correct. If any# change, BC,fps, attitude, DA, then the hold changes. It's math, it doesnt lie (most of the time). Strelok is only as good as the #we put into it.

Real question is how do bullet makers come up with a BC? All other thing change, I get that. But BC is a number, that bullet markers say is right. If I change BC and not DA then hold comes out right. We,are taking a # and not changing it (BC) and believe it is correct. How is that measured? In a controlled environment ? Or at a place that difference from place to place?
 
So to any pilots out there... How does DA affect flight? Thinker air requires more power to stay at same altitude, less DA requires less power?

Same as bullet in flight? Maybe thinner air reduces drag on air craft, thinker air makes more drag? If so...I think I'm correct on this... How do bullet markers come up with a BC?

Thinking about all that affects drag(drag=BC) Then BC has to be done in a controlled environment. If not then published BC is (maybe) not correct. If any# change, BC,fps, attitude, DA, then the hold changes. It's math, it doesnt lie (most of the time). Strelok is only as good as the #we put into it.

Real question is how do bullet makers come up with a BC? All other thing change, I get that. But BC is a number, that bullet markers say is right. If I change BC and not DA then hold comes out right. We,are taking a # and not changing it (BC) and believe it is correct. How is that measured? In a controlled environment ? Or at a place that difference from place to place?

Wow...........there's a lot to address here.
First of all, B.C. is NOT the same as drag. It's called a Ballistic Coefficient so that you can predict the drag, and therefore predict where your bullet will hit on the target if you factor in things like range, muzzle velocity, etc. The "etc." includes atmospheric variables.

To convince yourself that B.C. is not drag think about this. A bullet sitting on your loading desk has zero drag. Why? Because it's not moving; however, it has the same BC that it does when it's halfway to the target when you shoot your next match.

When people calculate B.C. they typically measure the actual bullet performance and then work backwards using the actual atmospheric conditions under which they did their testing to arrive at a B.C. which they then publish. Typically they don't test bullets or aircraft in a standard atmosphere. They test in the atmosphere at the time and location they have at the moment, and then correct the results to some standard, usually to a standard temperature and pressure. By correcting their measured results to reflect the altitude, temperature, and humidly during the test, they come up with a standardized B.C. so that you can find the solution you're looking by working the other way. That is to say, you start with their standardized B.C., take into account YOUR weather conditions along with MV, etc. and then you can predict where your bullet will land.

Density altitude is a favorite for pilots because it gives a number in feet above sea level rather than some other unit like slugs per cubic foot. Few people have a feel for what a slug per cubic foot feels like.

If you are at an airport 2000 feet above sea level you can expect your take off performance to be less than you would get at an airport at sea level. But if the weather is hot, your density altitude might be 2800 feet even though your "real" altitude is only 2000 feet, meaning that your take off performance will suffer even more. Of course there are performance charts pilots use to calculate their take off (or landing) performance and Density Altitude is usually one of the primary factors.

Density Altitude is normally easy for pilots to get since they most often do their work at an airport and airports are linked to weather observations and forecasts. We shooters are not so lucky, but we can often find our altitude and temperature which are the main factors. Humidity has less effect on D.A. but it should be counted if you can find out the value. Various ballistics calculators handle atmospheric conditions differently.

Contrary to your intuition, moist air is "thinner" than dry air. It may feel "heavy" to you if you're out in the swamp chasing alligators, but in fact moist air is less dense.

Now when it comes to the power required to fly an airplane in "thick" or "thin" air, the answer is a bit complicated to cover in this thread. Even though my formal education is in the field of Aerodynamics, I have no intention to turn this thread into an aerodynamics course. But I will say this: Generally speaking, lower density altitude (thicker air) requires LESS power to fly; just the opposite from your guess.
 
Well put. A reminder of my own days as a pilot.

Density Altitude has a profound effect on bullet flight, and the performance (and accuracy) of acoustic electronic targets also. I am having difficulty in getting this message across...

A lot of it has to do with basic physics.

I have found myself explaining to some shooters why in hot weather they find themselves lowering their elevation slightly (due to lower air density). And also why the measured error (area of uncertainty) of the ET has increased accordingly (but is only one of the factors governing this).

I regularly have shooters shooting at 2000 ft but have a DA of over 4000 ft (in the summer in 95+ degF or 35 degC heat) and don't understand why things have changed on them until it is pointed out that for every extra degree Celcius the altitude increases 120 ft. After some explanation the penny drops.
 
Shootdots,

So why do you think the load changed from TX to AR and stayed the same when back in TX?

I'm guessing that my crono was correct at 3104(average) but due to DA the BC of the bullet was different? So how do the bullet makers come up with a BC of a bullet? Do they shoot them in controlled lab? Or outside where everything changes? Maybe I'm over thinking this and just need to figure DA for everytime I go shooting, but would like to understand how this works.
I do understand how it works, you are getting good info here, I no longer even think about it today, I use this, a solid zero, real muzzle velocity, plus most bc's in the bullet library close. For 700 bucks, best thing I've ever bought for LR shooting:
https://kestrelmeters.com/products/kestrel-elite-weather-meter-with-applied-ballistics
 
We approach shooting and density altitude from a different direction as we shoot short range (100-200 yards). I have kept notes of what to expect on the target while shooting 118 grain 30mm Bib Bullets on top of 34.0 grains of H4198. Those note include what happened at a particular density altitude on a given day and where the Beggs tuner was set. Those notes are for a particular barrel on a particular rifle, too. I check the DA before each and every target as the DA changes that much, that often and I twist the tuner according to my notes. Before I go to the range, I enter the altitude of that range into the Kestrel 4000 as that one variable will affect the true density altitude.
Now, I checked with Neilson-Kellerman to see if this old meter was up to date. Sure enough, my Kestrel 4000 has been discontinued and replaced by the Kestrel 5000. In true tradition, N-K will allow me to trade my old 4000 in for a new 5000 at a discounted rate.
 
Wow...........there's a lot to address here.
First of all, B.C. is NOT the same as drag. It's called a Ballistic Coefficient so that you can predict the drag, and therefore predict where your bullet will hit on the target if you factor in things like range, muzzle velocity, etc. The "etc." includes atmospheric variables.

To convince yourself that B.C. is not drag think about this. A bullet sitting on your loading desk has zero drag. Why? Because it's not moving; however, it has the same BC that it does when it's halfway to the target when you shoot your next match.

When people calculate B.C. they typically measure the actual bullet performance and then work backwards using the actual atmospheric conditions under which they did their testing to arrive at a B.C. which they then publish. Typically they don't test bullets or aircraft in a standard atmosphere. They test in the atmosphere at the time and location they have at the moment, and then correct the results to some standard, usually to a standard temperature and pressure. By correcting their measured results to reflect the altitude, temperature, and humidly during the test, they come up with a standardized B.C. so that you can find the solution you're looking by working the other way. That is to say, you start with their standardized B.C., take into account YOUR weather conditions along with MV, etc. and then you can predict where your bullet will land.

Density altitude is a favorite for pilots because it gives a number in feet above sea level rather than some other unit like slugs per cubic foot. Few people have a feel for what a slug per cubic foot feels like.

If you are at an airport 2000 feet above sea level you can expect your take off performance to be less than you would get at an airport at sea level. But if the weather is hot, your density altitude might be 2800 feet even though your "real" altitude is only 2000 feet, meaning that your take off performance will suffer even more. Of course there are performance charts pilots use to calculate their take off (or landing) performance and Density Altitude is usually one of the primary factors.

Density Altitude is normally easy for pilots to get since they most often do their work at an airport and airports are linked to weather observations and forecasts. We shooters are not so lucky, but we can often find our altitude and temperature which are the main factors. Humidity has less effect on D.A. but it should be counted if you can find out the value. Various ballistics calculators handle atmospheric conditions differently.

Contrary to your intuition, moist air is "thinner" than dry air. It may feel "heavy" to you if you're out in the swamp chasing alligators, but in fact moist air is less dense.

Now when it comes to the power required to fly an airplane in "thick" or "thin" air, the answer is a bit complicated to cover in this thread. Even though my formal education is in the field of Aerodynamics, I have no intention to turn this thread into an aerodynamics course. But I will say this: Generally speaking, lower density altitude (thicker air) requires LESS power to fly; just the opposite from your guess.


This is the info I was looking for! Thank you.
 
Remember also as the bullet goes down range its BC changes as it looses velocity. Some programs calculate for this some don't. That's why BC charts like the one in the Sierra Manuel list bullet BC's at different velocities.
 
I'm probably gonna get flamed...but here goes anyway... I don't use density altitude. There, I said it.

DA is absolutely necessary for a pilot because it takes into account both heat and humidity as well as station pressure. Both heat and humidity affect how efficiently an engine consumes fuel as well as the efficiency of the propeller or rotors actually beating through thinner or thicker air. All this occurs over an extended period of time and is important for both instantaneous take off power as well as it's cumulative effects over the course of the flight.

However, for a projectile with a "flight" under 2 seconds, all you really need be concerned about is station pressure. The "heat" part of the equation is figured in when you adjust velocity to account for your powder burn rate. The powder burn rate will be different for every cartridge / powder combination. A 223 will not be affected the same as a 50 BMG in the exact same conditions. Humidity means next to nothing except that it is already somewhat figured in when the correct station pressure is used.

The practical aspect is that I don't want to depend on an instrument that can fail to either provide or calculate DA and I don't want to carry one more chart in my data book. My G-shock gives accurate station pressure, and if I know my altitude, I can just plug in standard pressure for that altitude and probably be within 0.5 of Hg anyway. I can carry a simple thermometer in my pack to give me temperature. Simple is better and nothing extra to carry or fail in the field.

When wind calling, the standard pressures at the altitude you are shooting at is more than close enough to adjust wind drift. So...I said it. Flame on.
 
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