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

 

[winmag]

Im guessing folks like Broz, Bigngreen, Eaglesnester, and Boomtube would be able to shed light on the more scientific "why's, and how to compensate" questions.
I havent noticed groups opening up at elevation, mostly because I havent shot groups when I get where Im goin, plus my loads are developed at 3800 feet and I usually hunt in the 3500-6000' range, with few exceptions into the 8-9000' range. However I doubt elevation changes anything in the way of harmonics, but rather air density, and pressure so you shoot a bit flatter at higher elevation cause the air is thinner. I run JBM,(knowing JBM is a bit optomistic) then shoot to confirm drops out to my farthest yardage I feel comfy shooting. In my case my shooting ability and equipment start leaving me wanting more at around 850yds even under ideal conditions, so Ill confirm 0 at 200, then drops at 400, 600, and 800, and run with it. I cant consistantly shoot accurately past there anyway, so I usually have very little fine tuning to do, but those that can should be able to shed alot of light on this question. I look forward to learning so I can put it into practice as my L/R equipment and ability continue to grow.

 

[SBruce]

I can't speak from experience on groups opening, because I've always shot at nearly the same altitudes (not the 6 to 8 K feet you mentioned anyway) But I do know for a fact that the higher altitudes will change your sight in.

A friend recently brought his new Edge out from Illinois. It was zero'd exactly at 100 yds in Illinois, but out here; it was 3/4 inch high at 100 yds. The drops out to 600 yds had been confirmed too in IL, but changed once here. (We're assuming that the scope didn't get banged in transport) Even after changing the distance the rifle was zero'd for (in the program) drops still didn't quite match the ballistic programs. We're still trying to figure that part out!!

I doubt that many folks are changing their load because of altitude, and am too curious as to if any bring their chronograph along.

 

[royinidaho]

Elevation, in and of itself, will not change 'load' characteristics.

Temperature will.

The difference in air density as elevation increases changes the bc of the bullet from where it was from a lower elevation. This is usually not enough for a miss or poor shot out to say about 700-800 yards. Beyond that it becomes an ever increasing factor with distance.

Most check zero at or near the new elevation and rely on developed drops from there.

The greater the zero distance the better things will be, up to a point. All my shots are long so I zero @ 200 yds for a flat shooter and 300 yds for a rig that shoots rainbow trajectories.

If drops don't match at the higher elevation, Ill to WY, look at what was done to make the chart match the drops. The method used may or may not have been the most appropriate.

I was told once, by one who I'm pretty sure knows, that a good workable rule of thumb is the MOA drop decreases about 0.1 MOA per 1000 ft of elevation increase. This has seemed to work when I tried it.

I live, shoot and zero at 4450' and hunt and shoot rocks from 5500' to about 8500'.

 

[Len Backus]

MOA drop decreases about 0.1 MOA per 1000 ft of elevation increase.

I don't think that is accurate, Roy.

 

[royinidaho]

Len,

I'll stand corrected on that one.

Was going from memory from a discussion w/BB on that hunt in '07.

When I tried it I was shooting a mil dot reticle and "mentally" converting MOA to mils. That could be the fly in the ointment.

Thanks for keeping things on the up and up.