Zero compensation at altitude

[journeyman713]

Does anyone have a good reference for predicting zero 200 yards at altitude, before you get to the altitude?

Of coarse there will be reverifcation later but...
For example, to be dead on Zero at 8000 feet, should I be shooting at say.... a half inch low at 700 feet?

The ballistic calculators I found seem to not connect the variation in elevation without a rezero.

Last trip as I recall, most guys in our group were having to adjust for shooting high zero. Just trying to plan one step ahead. Thanks

 

[ChrisAU]

Zero at 100 yards to help eliminate environmental variables to your zero. Then once you've trued up your ballistics with known drops, you can be confident in your chosen calculator when you change the elevation/pressure/temp etc.

 

[Carr5vols]

Nikon's spot on BC takes into account elevation ok n. I dont use there scopes but there BC is pretty nice.

 

[LaHunter]

For my 7 mm rem mag, I am zeroed at 200 yards at approximately 100 ft above sea level (Louisiana).
According to my Ballistic app, I have a little less than a .5moa difference in POI at 400 yards at 100 ft ASL vs 8,500 ft ASL. At 500 yards, the difference is just under .75 moa.
I think most apps will adjust for the altitude without needing to re zero your rifle. You just have to change the environmental conditions. The biggest difference is likely to be shooting at angles in the mountains. This will have more effect on your POI vs altitude alone.

 

[Formidilosus]

Guys, read what he’s asking. He’s not asking about distant shooting, he’s asking about a 200 yard zero and his zero changing due to elevation.

Does anyone have a good reference for predicting zero 200 yards at altitude, before you get to the altitude?

No. There is about a bullet diameter difference from sea level to 8,000 feet altitude at 200 yards. If someone is having to rezero at altitude, it isn’t from the altitude- the scope lost zero.

As above, there are many reasons to zero point of aim, point of impact at 100 yards. A static zero from sea level to Mount Everest is but just one.

 

[Ryan Avery]

I agree with Form

Every year I hunt from 2250 FAS to up to 11,000 FAS with no zero change.

 

[Journeyman]

I think the change in zero that a lot of guys experience has to do with multiple things, none of which I believe is due to elevation.

First off is possibly equipment malfunction.

Next, most guys zero off a bench then check zero during their hunt prone off a bipod, rested over their pack etc. This can cause a perceived change in impact with poor fundamentals and a light hunting rifle.

Another big problem I've seen is guys not knowing how to properly align their eye behind a scope without a parallax adjustment.

I agree with zeroing at 100 then if you want your rifle set at 200, 250, 300 etc. just dial up and leave it there.

 

[ChrisAU]

I think the change in zero that a lot of guys experience has to do with multiple things, none of which I believe is due to elevation.

First off is possibly equipment malfunction.

Next, most guys zero off a bench then check zero during their hunt prone off a bipod, rested over their pack etc. This can cause a perceived change in impact with poor fundamentals and a light hunting rifle.

Another big problem I've seen is guys not knowing how to properly align their eye behind a scope without a parallax adjustment.

I agree with zeroing at 100 then if you want your rifle set at 200, 250, 300 etc. just dial up and leave it there.

Yep, I can zero at 100, then dial up 1.25 MOA for a 200 yard zero at both 500' and 10,000'. But then I have drop charts for further ranges at the elevations that I hunt most frequently, and they are less effected by environmental factors than a 200 yard zero would be because right off the bat my 200 yard zero would be off 1/10 MOA going from 500' to 10,000'.

 

[journeyman713]

Great feedback, it seems the consensus is the variation is minimal, with factors other than elevation being the cause.
This got me thinking that the problem of shooting a high zero at altitude, may be more prevalent with the guys that like to shoot a little high to begin with at home. This is a club I used to be in, figuring up to an inch high is good for zero. Then go to elevation and find the one inch threshold has been crossed - readjust. In reality, the actual change is quite small.
This year I'm going with the Leupold Custom dial system and want things exact, as my comfort shooting range has increased. Two weeks away!

 

[PONYBOY]

Does anyone have a good reference for predicting zero 200 yards at altitude, before you get to the altitude?

Of coarse there will be reverifcation later but...
For example, to be dead on Zero at 8000 feet, should I be shooting at say.... a half inch low at 700 feet?

The ballistic calculators I found seem to not connect the variation in elevation without a rezero.

Last trip as I recall, most guys in our group were having to adjust for shooting high zero. Just trying to plan one step ahead. Thanks

Hornady's app is excellent. I just zero at 100 to make easier.

 

[Newt]

Great feedback, it seems the consensus is the variation is minimal, with factors other than elevation being the cause.
This got me thinking that the problem of shooting a high zero at altitude, may be more prevalent with the guys that like to shoot a little high to begin with at home. This is a club I used to be in, figuring up to an inch high is good for zero. Then go to elevation and find the one inch threshold has been crossed - readjust. In reality, the actual change is quite small.
This year I'm going with the Leupold Custom dial system and want things exact, as my comfort shooting range has increased. Two weeks away!

It will be minimal, but there will be a bit of a difference. I agree with the 100yd zero club. It will make the difference almost nothing.

You can calculate it. Just run the numbers with a 200yd zero at your current altitude, note what the elevation(rise) is at 50yds, then run the same data but at the altitude, you are going to be at, and change the zero range until the 50yd data matches up. This will get you really close to knowing the exact zero. I actually used the data for 30-50 yards, in 5-yard increments.

The only effect altitude has on ballistics is air density. Of course, most of the time temp and stuff will change also, but sometimes not depending on when you zero your guns. Air density has its effect on a projectile the longer the projectile is in contact with it. That's why unless you have a 500-yard zero or something crazy, you'll never see much of a difference in zero.

However, if you are shooting great distances you can quickly see the differences in trajectory with the minimal difference in zero. For instance, using my .308 load, all things equal but elevation, going from 700' to 8000' puts a 200yd zero at 700', to 205yd zero at 8000'. That's not much really. However, once you stretch that out to 500yds, its a 5" difference. That, in the precision game, is quite a bit.

Honestly, my opinion, the difference in zero is not enough to be concerned with unless you for sure going to be shooting really long distances and are going to be shooting something that has a relatively low velocity and has a low BC value.

 

[ChrisAU]

The last couple of episodes of the Hunt Backcountry podcasts with Ryan Cleckner as guest are excellent for getting a good base understanding of ballistics and altitude.