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

 

[4xforfun]

Since no one else is going to chine in, I will. I have zero experiance with shooting different altitude...so take that for what it's worth.

Altitude should have no affect on internal balistics......all of the changes occure after the bullet leaves the BBL.....altitude, temp, and baro. With no change in internal balistics, your muzzle velocity should be the same.


Now, that being said, your high altitude zero will be different than your sea level zero....you must re zero at altitude. Your post never said that you re-zero...that could be your problem.

Good luck.

 

[djtjr]

thanks i do re zero for altitude as i know thats part of the equasion but it only seems to be a portion of the issue, i am just trying to account for all of the variabbles. this is for a long range hunting gun so its not like i can easily tweak the load to adjust as i could at a bench. the yardage the altitude and temp will vary shot by shout hour by hour so i am just trying to account for all the variables as best i can once i know what i can do to get the best shots possible i will then limit my range to what my gun load and my own abilities can handle.
anyone else with some experience with altitude?
d

 

[15Tango]

I use Sierra's ballistic program you can enter a multitude of environmental factors.

Altitude has a great effect on trajectory, the higher you go the flatter the trajectory as the drag decreases.

Here's an example:

142 SMK @ 2800 FPS at 1000 feet AGL, 100-yard zero, drops 290 inches at 1000 yards.

Now everything else being the same but at 10,000 feet AGL it drops only 252 inches.

Your zero won't change much at either height, the range is too short. With a hundred yard zero the difference between 1000 and 10,000 feet at 200 yards for drop is only .1"

With a program likes Sierra's you can get it all dialed in and know what it will do at 8000 feet versus 1000.

Wayne

 

[glo]

In my experience altitude has no effect on my zero; zeroed in Los Angeles at 1200ft. and hunting in Gunnison, CO at 10,000ft..

The change in impact comes from the extreme temperature change; 80 degrees to 30 degrees. The powder burning rate slows way down. I now use only Hodgdon Extreme powders to limit the affects of temperature on my internal ballistics. Also the up hill and down hill shooting. I zero at nearly zero degrees. Most of the shots I’ve taken in my life are either 5 plus degrees up or down. I’m rarely ever on flat ground.

P.S. There are two other factors that will change you zero. The first is a cleaned and well lubed barrel. Never take a clean rifle Hunting. Always work up your loads, clean the rifle and then shoot 2 foulers. Otherwise your first cold bore shot will be high and to the right.

The other reason is if your rifle has a mass produces, hammer forged, chromoly, noodle barrel. Good luck ever holding a zero.

 

[15Tango]

The change in impact comes from the extreme temperature change; 80 degrees to 30 degrees. The powder burning rate slows way down.

That's the subject of internal ballistics only, the OP is seems to be concerned about that and external ballistics as well.

If you want to find out what your velocity will be at a given temperature you could take a cooler with ice in it and a thermometer in it, leave your cartridges in the cooler and see how much of a difference the temperatures are effecting your burn rates and velocities.

However, external ballistics at higher altitudes have a lot more to do with air density than temperature. Using my previous example of a 142 SMK at 2800 FPS, at 1000 yards and 1000 FT AGL the difference of drop between 80 degrees F and 30 degrees F only accounts for about 14" difference in drop, whereas the density altitude change between 1000 and 10,000 AGL accounted for 40".

Wayne

 

[4xforfun]

to djtjr -

Density Altitude affects is what your asking about. Here is my input.

For Internal ballistics = effects the powder burn rate, which effects pressure/velocity, which effects accuracy and trajectory.
For External ballistics = effects Ballistic Coefficient, which effects Time in Flight, which effects trajectory

At 100yds, a change to a Zero of 0.050" will change trajectory with some significance. Especially at long distances (say to 600yds or more).
A 0.100" change to a Zero at 100yds will change trajectory very significantly.............
Most people do not realize the significance of a little change at 100yds to trajectory down range. Especially at long distances.
I suggest a 300yd "True Zero" is a better and more accurate Zero for accurate trajectory predictions.

Your description to seeing a lose of accuracy from your sea level environment to when you go west to 6-8000', is do to the internal ballistic effect air density is effecting your pressure & velocity.

Happy Shooting
Donovan Moran

Hey D...

I just checked my Infinity program. Berger 210 vld @3220 fps, 20 degrees F, 1500 feet alt, 78% humidity, 29.53 baro....scope hight 2.1"

100 yard zero = 209.23" low at 1000.
90 yard zero (a change of exactly .05" at 100) = 208.68 low at 1000

Less than 1 inch difference. Hardly "some significance"

changing the zero to 85 yards ( .100 inch off at 100) = 208.2 inches of drop....about an inch......hardly "very significantly"..

 

[4xforfun]

Hey 4xforfun -

Your changing the wrong end............ Do you plan on making that 1000yd shot with your 100yd Zero?
Look at the "bullet path" in the opposite way. Depending on what cartridge/bullet/velocity/etc... 0.100" less at 100yds will move your 1000yd POA, impact some where around 965 to 985yds.

The way your looking at it you could be 1" off your 100yd-Zero and still be with in 10" at 1000yds.
Sorry....it don't work that way.


>>>....Later....PS: Is this the infinite Ford salesman from just shy of the Canadian border?

Happy Shooting
Donovan Moran

Yep, it's me.

Now...I know that you are a numbers guy, who also has about a million rounds down range.....me....I think I skipped that day in school.

My question to you is....the numbers you gave...965 to 985.... are these the yardage the bullet strikes the ground with an aiming error of -.0 something with a 1000 yard zero?

Because if it is, the flatter the trajectory of a bullet is, the further the miss is going to be and visa versa. If you take a +.1 error in aim....the bullet with a very very low bc/fps, the bullet will land very close to the intended poa.....the ultra flat shooting gun, with the same intended POA and the same + .1 error in aim point, will hit WAY, way behind the target. .

This all, of course, is getting way off topic. He is not asking about point of aim errors.

My thoughts are this...if he is having a problem with changing velosity due to eleivation (which I don't believe is possible.....the change is an internal balistics thing due to the powder reacting to the temp change....most likely) he needs to leave a day early, bring his chronograph along, a couputer program, a laptop, and some extra ammo. Get your real world velosity and zero, figure up computer generated drop chart, and then pick a distance and test it. If you are off at 800 or 1100, you will be off everywhere. Tweak the numbers (bc and fps) to get the computer to spit out the same numbers you are getting on paper. If the temps change a lot where you hunt, or the altitude changes dramaticly from spot to spot where you might hunt, then use an average temp/alt for your imput.

You could also try the cooler thing with the cold bullets, but that won't give you any piece of mind if you still think it is an altitude thing......practice where you hunt.

Next....write EVERYTHING down....for next years hunt.

That is all the advice i can give, given my lack of experiance and, well due to the fact that I skipped a lot of school!!!! ;D

Let us know how things turn out.

 

[FBecigneul]

Your groups opening up can be attributed to the twist rate of the barrel. You need to see what Dan Lilja says about using a different twist for variations in altitude. A longer or shorter bullet will compensate for some of that as well. You are going through an altitude change of 6000-8000 feet and that is considerable.

 

[people]

Kind of off topic but a

 

[kemen4]

Try ...
http://www.exteriorballistics.com/ebexplained/5th/31.cfm

 

[RDavies]

You asked about effects on loads, not trajectories and I think I might have a bit of an idea. (didnt say it was right).
The density of the air in the barrel, if the temerature is the same, will be lower. So if it is a relatively warm day up on a mountain and you rifle was tuned in similar temp down near sea level, there will be less air density for the bullet to react against inside the barrel, which might change the barrel time ( and hence what time the bullet will exit the barrel in the vibration cycle).
Does anyone out there who actualy knows this stuff,know if this has some merit or is a load of bull?