Cnkhunting
Well-Known Member
How much difference in zero can altitude make? I live at 4400 and hunt from 8-11000 feet.
Would it make my groups shoot lower or higher?
Would it make my groups shoot lower or higher?
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Altitude and zero
- Thread starter Cnkhunting
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Cnkhunting
Well-Known Member
I have never noticed it till this year with my 7mag. I zero it at about 4800 feet and shot groups at about 6800 feet and the groups were about 1.5" lower. The way I am thinking is lighter air at higher altitudes it would shoot higher. Some one explain this please.
Hand Skills
Well-Known Member
Temperature can be a factor too - did you test in the same conditions you zero's under?
Cnkhunting
Well-Known Member
With 5-10 degrees
entoptics
Well-Known Member
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- Jan 16, 2018
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- 880
A bullet will drop less as elevation and/or temperature increases because air density decreases.
Unless you're zeroed at a heck of a long ways out, or using a very slow bullet with a terrible B.C. (45 ACP?), elevation will have a negligible effect on your zero point of impact. I ran the numbers from 2000-10000 ft, and it's less than or equal to ± 0.1 inches at 200 yards at either extreme. At 600 yards, it's about 0.5" - 1" per 1000 feet of change depending on B.C. and muzzle velocity.
For temperature, it's ~1/3" per 10° at 600 yards, and too small to register on JBM ballistics at 200 yards, because it's less than 0.1 which is all the decimals displayed.
Inputs...
0.275 G7 B.C. (~0.550 G1 B.C.) @ 2900 fps
Zeroed at 5000 ft, 29.92 sea level corrected pressure, 70° F, 40% RH.
Zero angle was fixed at the above zero conditions (e.g. you zeroed your rifle once, then shot that same zero in the variable conditions shown below without rezeroing to 200 yards again at the different conditions).
In summary, if your zero was off by 1.5" it's got nothing to do with temp or elevation, and is a result of some other issue.
Unless you're zeroed at a heck of a long ways out, or using a very slow bullet with a terrible B.C. (45 ACP?), elevation will have a negligible effect on your zero point of impact. I ran the numbers from 2000-10000 ft, and it's less than or equal to ± 0.1 inches at 200 yards at either extreme. At 600 yards, it's about 0.5" - 1" per 1000 feet of change depending on B.C. and muzzle velocity.
For temperature, it's ~1/3" per 10° at 600 yards, and too small to register on JBM ballistics at 200 yards, because it's less than 0.1 which is all the decimals displayed.
Inputs...
0.275 G7 B.C. (~0.550 G1 B.C.) @ 2900 fps
Zeroed at 5000 ft, 29.92 sea level corrected pressure, 70° F, 40% RH.
Zero angle was fixed at the above zero conditions (e.g. you zeroed your rifle once, then shot that same zero in the variable conditions shown below without rezeroing to 200 yards again at the different conditions).
In summary, if your zero was off by 1.5" it's got nothing to do with temp or elevation, and is a result of some other issue.
What distance was your zero? This is one of the advantages of 100 yard zero (reducing environmental variability)
But as said 1.5" is a lot. Makes me wonder more about a zero shift especially just going from 4500-6800 that's not a huge difference
But as said 1.5" is a lot. Makes me wonder more about a zero shift especially just going from 4500-6800 that's not a huge difference
Last edited:
Cnkhunting
Well-Known Member
It was at 100 yards. It was the range I zeroed at. It's not flat. It's an up hill to target. So that's where I got screwed up.
