Accuracy at high altitude

[Dr.T.]

Hi all - I recently heard that loads tuned for low altitude will not shoot as well when shot at high altitude (the discussion was in respect of F class competition shooting, but I think the principles would apply more generally). The reasoning given was that the air is thinner, and bullets retain more velocity...

I appreciate that high altitude, and lower air density, will alter POI. However, I don't follow the speed argument because a tight shooting rifle shoots tight no matter the distance (and therefore different speeds, within limits of course), and I can't imagine air density having a noticeable impact on powder burn, case pressure or barrel harmonics (but I might be wrong).

Theory aside, has anyone actually experienced a change in group size when shooting at higher altitude?

Many thanks.

 

[Dusty Stevens]

Try it in your favorite ballistic calculator

 

[ballisticdaddy]

I have shot for many years in f-class long range matches at less than 2,000 feet. Earlier this year moved to Colorado and besides for having much less elevation dialed into my scope the previous established loads in two of my rifles shoot just as well at over 6,000 feet as they did before.

 

[Dr.T.]

Thanks ballisticdaddy that's what I suspected.

Dusty Stevens- I was referring to altitude effects on group size, not POI (unless there is something in a ballistics calculator that I'm missing)?

Thanks again!

 

[XTR]

I know a lot of guys who tune their F-TR loads around Houston and Lafayette, one of them won the F-TR national championship at Raton.

You are worrying to much. read the wind.

 

[mgunderson]

In my experience more people have issues going from high elevation to low at the 1000 yard line. Loads that were supersonic start getting closer to the transition point or drop below.

Never had issues going higher or lower in elevation..... but temperature can show up.

 

[lukehalee]

I live/tune at an elevation around 600 feet. Can drive to Raton NM(elevation 6000+ ?) and don’t need to change anything. Hope this helps.

 

[rsmithsr50]

I live/tune at an elevation around 600 feet. Can drive to Raton NM(elevation 6000+ ?) and don’t need to change anything. Hope this helps.

4000 or so a the range

 

[lukehalee]

Thought it was higher. Thanks.

 

[XTR]

Raton is 6700 feet give or take

The runway at the airport just up the road is 6352

 

[Dr.T.]

Thanks guys - much appreciated!

 

[Ned Ludd]

For many people traveling from other parts of the country, the increased elevation at Raton usually works in their favor. The increased wind...not so much.

 

[searcher]

As you note - there WILL be, at best, vertical impact shift due to velocity difference from thinner air. As to losing tune? I have dope for two shooting locations - one at 300 ft. elevation - and the other 4,800. Shooting these two locations a lot with numerous guns, I have never noticed any change in tune that was not assumed to be due to air temperature or humidity - as they changed from trip to trip. It's my opinion that anyone who believes they have tuned their rifle to a particular atmospheric condition will be disappointed on a weekly basis if they expect it to remain the same. And that is shooting at the SAME place.

 

[Twoboxer]

A 30-06 load I developed at close to sea level showed an average MV of 2887fps on my Labradar when the temperature was in the 70s.

I then used that load on targets from 100 to 1200yds at ~2300’. The temperature was in the mid-70s, and the Labradar showed an average MV of 2915fps. I did not record humidity data.

I always assumed that lower pressure at the higher elevation would allow the powder charge to generate more pressure than at sea level.

I did not see a change in group size.

 

[Ballisticboy]

The change in air density in going from low to high altitude will change more than just the velocity drop of the bullet. The gyroscopic stability of the bullet will increase and there will be a change in the dynamic stability. The changes in stability will affect the bullet yaw and how quickly it damps out. If this effects your group size or not will depend on how marginal the behaviour of your bullet was to start with. If you are firing a bullet with low bullet yaw and satisfactory stability to start with, usually needed for small group sizes, then it is unlikely you will notice any change in group size. Problems, if present, will normally occur when going in the other direction from high to low altitude, particularly if the temperatures at low altitude are also low, when the air density increases reducing gyroscopic stability.
So if you have a setup which is already producing good results I would say you are unlikely to notice a large change in group size when firing a higher altitudes unless you are going very high.