Range Report effects of altitude on loads

[djtjr]

hey guys i am wondering if anyone has any experience with this but i am based in the east at sea level when i have gone out west to altitude (6-8000 feet) i have noticed that a) my groups open up a bit and b) balistic trajectories are different even when compensating for them with a balistic calculator in my case bullet flight. how do you guys eliminate most of the variables without just developing a load at altitude as that is not all that easy or convenient? does the lower presure at altitude also change the velocity of the bullet therefore changing the barrel harmonice and trajectories beyond just the effects during flight which a program should calculate for? if so do you bring a chrono when going to these high places? whats the best way to compute altitude altitude (ie changing altitude or actual presure or both in your balistic program) if you do presure and altitude could it be a double count that throws you off further? what do you do in your programs to account for the base case sight in at sealevel vs the new condition at altitude? i think you may get the idea of where i am going wth this but want to learn moea bout these variables for upcoming hunts.

thanks and any and all thoughts or perferably facts are appreciated.

best,

Don

 

[kraigWY]

Re: effects of altitude on loads

Lets make this practical:

I live at 4500 Ft, I Elk hunt at 10,000 ft. I use a Model 70 in 270 w/150 grn bullets sighted in at 250 yards.

I limit my hunting to 300 yards. So at home, my 270 drops 4 inchs at 300 yards. I go to the Big Horns, my bullet will drop 3.75 inches. That's well within Minute of Elk.

However, the correct answer would be a Data Book. Let's assume everything is the same except I want to shoot at 1000 yards (which I would never do hunting). Then at 10,000 feet my round would drop about 30 inchs (or 3 MOA) less then it did at home.

That's where the data book would come in. In it I would have the corrections logged. In looking at some of my old Score books where I was shooting 1000 yard matches, I do note there is a differanc between my zeros from when I shot 1000 yard matches at 29 Palms and Ft Lewis Washington. I didnt use computers and balistic tables that much back then, I relied on score or data books.

In normal hunting conditions, I wouldnt worry about it since my normal hunting conditions is < 300 yards.

 

[ReaperDriver]

Re: effects of altitude on loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: djtjr</div><div class="ubbcode-body">hey guys i am wondering if anyone has any experience with this but i am based in the east at sea level when i have gone out west to altitude (6-8000 feet) i have noticed that a) my groups open up a bit and b) balistic trajectories are different even when compensating for them with a balistic calculator in my case bullet flight. how do you guys eliminate most of the variables without just developing a load at altitude as that is not all that easy or convenient? does the lower presure at altitude also change the velocity of the bullet therefore changing the barrel harmonice and trajectories beyond just the effects during flight which a program should calculate for? if so do you bring a chrono when going to these high places? whats the best way to compute altitude altitude (ie changing altitude or actual presure or both in your balistic program) if you do presure and altitude could it be a double count that throws you off further? what do you do in your programs to account for the base case sight in at sealevel vs the new condition at altitude? i think you may get the idea of where i am going wth this but want to learn moea bout these variables for upcoming hunts.

thanks and any and all thoughts or perferably facts are appreciated.

best,

Don

</div></div>

I think the problem you're having is that the data you're feeding Ballistic is wrong. I say that only because I was in your shoes last year and I wasn't changing my altitude at significantly as you are. But the problem lies in the <span style="font-style: italic">type </span>of data you enter into the calculator. If you're letting the handheld iphone or whatever auto-populate the temp and baro pressure from the internet - its going to be wrong unless you're standing on the spot or very near to it where the data readings were taken from. What the program is TRYING to eventually calculate for you is Density Altitude (DA). DA is a function of humidity, temp and pressure. If you input those three data points taken at an airfield at 5500 MSL and then go shoot at 8000 MSL - your ballistic data will most likely be wrong because its rare that the DIFFERENCE between you and where the measurements were taken is goign to change at a prdictable rate - which is all the program is attempting to do. Its interpolating the measurements to YOUR position based on some assumptions built into the program that are most likely NOT going to be accurate.

If you want your data to be closer to your actual dope - you'll need to know the actual absolute pressure (station pressure) of where you're standing and not a sea level corrected baro pressure (which is what you get off the internet weather stations). You'll also need to know the temp and humidity of where you're standing as well. You can get this info from a handhel kestrel or a wristwatch that measures this stuff (I have a Suunto that works well for this). Now, when you plug that data into Ballistic - its going to ask if the pressure is absolute or not. If you're using a Kestrel, say "YES" and leave the altitude to zero.

You can also build some DA charts to take with you. Read this excellent link: Lindy's DA explanation

 

[sniperaviator]

Re: effects of altitude on loads

Right now I am taking the yards/1000 times 2% for a 1000' of altitude change.
So if the yards is 1000, the moa drop is 34, and the altitude is reduced by 1000' you get (1000/1000)* .02 * 34 = .68moa. So then you take 34moa and subtract .68 for a new moa at 1000 yards of 33.3 (which rounds to 33.25). Whew! Hope you can understand that.

For 500 yards which has an moa drop of 10.4 it would be (500/1000)* .02 * 10.4MOA = .104. The new moa for 1000' higher is 10.4-.104=10.3.
Not much of a change.

This is all preliminary. Does this jibe with what you guys are getting?

What they say to do is do a print out of your ballistics chart changing only the altitude and putting in the "standard temperature" for each altitude every 1000' to 12000'. So then you have a ballistics chart for every density altitude in 1000' increments. Whew that is a lot of work. They should get the computer program to do this automatically.

I suppose it affects your windage also.

 

[Greg Langelius *]

Re: effects of altitude on loads

I only use calc to get rough zero data, and use USCGS data, where available, to enter my altitude variable.

I'd not be sweating the effects of altitude on muzzle velocity. Such pressure differentials are woefully small change compared to issues like bore drag.

The effects of altitude/air density on external ballistics is very real and can be significantly pronounced. I have found that as long as I can get reliable data into my calc, the resulting zero data will get me pretty well into the ballpark.

But I would never depend on a calc to allow me to saunter into a completely alien operating area and deliver a first round hit. Not gonna happen.

Greg

 

[Lindy]

Re: effects of altitude on loads

Muzzle velocity changes only with temperature. The powder contains its own oxidizer, which is why guns will fire underwater.

Ballistic programs model air density pretty well.

ReaperDriver's suggestion to make sure you're feeding your ballistic program the correct data is a good one. You may find this reference useful:

Barometric Pressure and Ballistic Software

Sniperaviator wrote: <div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">They should get the computer program to do this automatically.</div></div>

JBM does. Just put in a list of the density altitudes you want. How to do that is explained here:

Making Ballistic Cards Using Density Altitude