What would cause this? Bullet drop with elevation change

[wbm]

His rifle zero has changed since you sighted it in, or he shoots the rifle to a different POI than you.

 

[wbm]

No sense in feeding the zoo.

 

[FEENIX]

 

[wkdwdg66]

Not talking just altitude density. Rule of thumb is 20% change in humidity is about .5 MOA. Increasing humidity leads to more efficient bullet flight, decreasing it will fall. So if he's at 80% and goes to 20% you'd come close to the 2" drop in POI he's talking about. But jumping up in altitude will increase the POI so I can't justify the whole 2".

Call me a newb if you must, but doesn't thicker air make bullet flight LESS efficient?

 

[sea2summit]

Call me a newb if you must, but doesn't thicker air make bullet flight LESS efficient?

Doesn't sound right at all does it? I was hoping Litz had an easy read on the internet I could find but no luck, here's another reference.

Ballistic Effects of Altitude, Temperature, and Humidity - Lilja

At a match recently I overheard another competitor comment that the wind seemed to be blowing the bullets more than usual that day. He suggested that the humidity level must be high, that the air was “heavy”. I know that there have been days when it seemed to me that a little change in the...

riflebarrels.com

 

[wbm]

His rifle zero has changed since you sighted it in, or he shoots the rifle to a different POI than you.

 

[NDF]

Haven't read all the posts, but the only thing that comes to mind is if he was shooting with the barrel only supported instead of as the original shooter sighted in off the stock or vice-a-versa, although the bourbon will have that affect also as previously documented

 

[Hegone9090]

We're one of you shooting west and one shooting east?

 

[wbm]

Good point.

 

[mmacfive]

Just throwing this out there. I've read all the reply's on temp. So certainly worth looking into. Different rest, could be. But I would throw out that powder will burn at different rate at 860 ft vs 7200. Especially if cold. So much less Oxygen at 7200. Rate of burn slows and velocity falls off slightly. Cold temp and less O2 to fuel the burn equals slower velocity in any textbook I've ever read.

I've always thought the case was a sealed environment?

 

[sea2summit]

I've always thought the case was a sealed environment?

Throw a handful of rounds it in a 5gal bucket of water for five minutes before firing and report back with results

 

[mmacfive]

I shot silhouette both handgun and rifle. Every single range has its own set of sighting adjustments. A slight difference in angle of shooting affects the sights adjustments. Elevation affects adjustments, temperature, humidity and altitude do as well.
The only thing I can think of for one of the rifles, the one you adjusted for your friend, not all of them...Is that if adjusted in humid weather, it shoots flatter. At high altitude or at altitude, (7200 feet is quite moderate) the air isn't as humid as at sea level or 850 feet. Humidity is air less dense and allows a bullet to fly with less drop. Dry air is more dense and creates more drop, although at altitude, rifles shoot flatter in general.
Regardless, you will not see 2 inches of difference at 100 yards however. We each pull the trigger differently. You adjust a rifle (scoped or otherwise) for someone else, you've only adjusted it for yourself and no one else.
I don't mean to be critical since we all at one time or another could do something similar for a friend, my point is that as has already been mentioned by others in this thread, there are too many variables in exterior factors and differences between one shooter and another.

FYI I just ran the calculations for 0% humidity vs 100% humidity at 1500 yards with one of my profiles. The was a 0.2 MOA difference so I'm not going to worry about humidity too much.

 

[wbm]

His rifle zero has changed since you sighted it in, or he shoots the rifle to a different POI than you.

 

[mmacfive]

Could be part of it but I'm thinking humidity plus temps. If he's down at 800' with 80-90% humidity then goes to 7k with 30-40% that could be about 2" difference depending on the cartridge but that would be on the high side at 100 yards.

Verify your humidity theory before you post stuff like this! I just run a profile for one of my guns at 1500 yards. There is a 0.2" difference when comparing 0% vs 100% humidity at 1500 yards. Or 0.01" at 100 yards.