Point of impact, consistency, and altitude change 1200ft to 9000ft elev.

[ILikeLead]

All my reloading and shooting occurs in Arkansas at elevations between 1000-1800 ft. As most handloaders do, I find the most accurate reasonable load as I work up a rifle-powder-bullet combination.

In a couple of weeks I am going to Colorado to a shooting range at about 7-8000 ft elevation. I know that less air resistance will equal slightly higher points of impact but can anybody with experience in altitude changes comment on how accuracy will be affected if at all?... Another way of asking this is would a different powder charge be more accurate at a different elevation or is it purely a function of that particular rifle?

Thanks in advance!

 

[ArchAngelCD]

Accuracy will probably remain the same or better. Less air resistance means your bullet will stay supersonic longer and not go through a transition theoretically increasing accuracy.

 

[Cemetery21]

I can't say about 7-8000, but I have chronoed varmint loads here (1100 ft) and NM @ 4100 ft and only got 25 fps difference. Pressure, POI, and accuracy was not affected that I could tell.

 

[Slamfire]

I suspect the temperature drop from higher altitude will have more influence on trajectory than air pressure. Might also have more influence on you depending on how physically active you are at altitude.

 

[Steve Cover]

I seriously doubt that you will be able to hold steady enough to tell the difference.
However, shooting up or down hill does change the trajectory over distance.
Rather than worry about the minute difference caused by higher altitude, you may want to explore the differences caused by different vertical angles.

Good luck with your hunt.

Steve

 

[jmr40]

It can make quite a bit of difference. Plug your numbers into one of the online ballistics programs. I used this to calculate my go-to 30-06 load of 165 gr Accubond @ 2880 fps at sea level.

http://www.hornady.com/ballistics-resource/ballistics-calculator

Shooting at 9000' there was 5" less drop at 500 yards. The same bullet had 350 more ft lbs of energy and was almost 250 fps faster than the numbers given at sea level.

 

[jmr40]

If you are using a powder such as H4350 that is resistant to temperature changes expect about .5 fps slower MV for each 1 degree lower temperature. With most other powders expect 1-2 fps slower MV for each degree temps change.

You can also calculate the drops with various angles by using the ballistics programs. With the Hornady version above you have to click on the "advanced" version.

 

[Steve Cover]

It can make quite a bit of difference. Plug your numbers into one of the online ballistics programs. I used this to calculate my go-to 30-06 load of 165 gr Accubond @ 2880 fps at sea level.

http://www.hornady.com/ballistics-resource/ballistics-calculator

Shooting at 9000' there was 5" less drop at 500 yards. The same bullet had 350 more ft lbs of energy and was almost 250 fps faster than the numbers given at sea level.

And you can hold that tight in the field to hold 5" lower at 500 yards?

That is a pretty small amount.
Like I originally said, I doubt that you could hold tight enough (in an actual hunting scenario) to actually notice the difference.

Just for grins, plug the same load data into a forty five degree down angle at 500 yards.
How does that compare?

[165 Gr. Accubond @ 2880 FPS (G7 Ballistic Model) .475 BC 275 yd. Zero..... Zero vertical angle = -27" @500 yards ... -45 degrees vertical angle = -9.8" @ 500 yards]

Over 17 inches different POI shooting down hill.
This is a big enough difference to try to compensate for in the bush.

Being mindful of vertical angle is much more important than altitude, barometric pressure or even temperature at the ranges a hunt is likely to be conducted at.

Steve