Estimating ballistics at altitude

[Newtosavage]

I could use some help with this one...

My home range is at approximately 300 feet ASL. Having said that, it's typically 80-90 degrees and very humid when I am zero'ing loads before heading to the mountains.

What adjustments should I expect to make when I arrive at say, 9K feet with air temps in the 30's-50's?

I have used the basic Hornady ballistics calculator for years, but have no idea what inputs to enter for barometric pressure at 9K feet, for example.

Last year, I had the chance to hit a range in NW Colorado during the scouting days prior to my hunt, and found that while my 162 ELD-X's grouped very nicely at 400 yards, they impacted about 4" higher than I expected.

Any basics you can share?

Thanks.

 

[RosinBag]

You won’t need any for your zero. Just build a ballistic chart to the elevation and environmental conditions you expect in your hunting area.

If you have a chance just try and shoot a mid range shot when you get there to confirm it all.

At 100 yards, environmental factors with a rifle will not affect anything.

 

[Newtosavage]

You won’t need any for your zero. Just build a ballistic chart to the elevation and environmental conditions you expect in your hunting area.

If you have a chance just try and shoot a mid range shot when you get there to confirm it all.

At 100 yards, environmental factors with a rifle will not affect anything.

That was my approach last year, and will be again this year (thank you BLM), but how do you figure barometric pressure when building a chart?

 

[sndmn11]

https://www.wunderground.com/history You could look up past weather data to find typical barometric pressure.

 

[RosinBag]

That was my approach last year, and will be again this year (thank you BLM), but how do you figure barometric pressure when building a chart?

The easiest to remember is basically every 1000’ take away 1 whole number. Sea level = 30.00 so 5000’ is 25.00. That will almost always get you very close minus extreme weather changes.

 

[kad11]

JBM calculator allows you to set a zero elevation and then create charts at a different elevation... I'd assume their algorithm uses the rule of thumb RosinBag mentioned to account for Bp

 

[Browninglover1]

Take your cellphone with you to run ballistics calculations live. Tons of phones have pressure meters on them and lots of rangefinders do as well.

 

[Newtosavage]

Take your cellphone with you to run ballistics calculations live. Tons of phones have pressure meters on them and lots of rangefinders do as well.

Oh goodness that's not something I need to be worried about when I'm hunting. I'm a 400 yard shooter, max. Just wanted to be a little more precise out to that distance. Essentially, I'll zero the tip of my bottom crosshair post at 400 yards. With my reticle that's 6 MOA below the crosshairs. My loads tend to go 3/6/9 MOA at 300/400/500 yards, so that's all very easy for me to remember when the blood is pumping. Anything inside of 200 and I hold dead on since my zero will usually be around 170-175 yards.

Mostly I'm curious about velocity at those distances, for opening up mono bullets, etc.

 

[Newtosavage]

The easiest to remember is basically every 1000’ take away 1 whole number. Sea level = 30.00 so 5000’ is 25.00. That will almost always get you very close minus extreme weather changes.

Thank you. that helps.

 

[Browninglover1]

Oh goodness that's not something I need to be worried about when I'm hunting. I'm a 400 yard shooter, max. Just wanted to be a little more precise out to that distance. Essentially, I'll zero the tip of my bottom crosshair post at 400 yards. With my reticle that's 6 MOA below the crosshairs. My loads tend to go 3/6/9 MOA at 300/400/500 yards, so that's all very easy for me to remember when the blood is pumping. Anything inside of 200 and I hold dead on since my zero will usually be around 170-175 yards.

Mostly I'm curious about velocity at those distances, for opening up mono bullets, etc.

At 400 yards I wouldn't be worried about baropressure or temperature. However, once you start shooting long it's worth it to make sure all your ducks are in a row because there is nothing worse than sending a bullet high/low that could have been dead on if you had the proper dial up.

Good luck on your hunt!

 

[RosinBag]

If 400 or even 500 is your max distance, none of these environmental factors will cause you to miss an animal.

 

[Newtosavage]

Yea, it's as much for my own curiosity as anything. Always trying to learn. That's why I ask!

 

[Newtosavage]

Here's more or less what I'm looking at for where I've worked up my loads vs. where I expect to be hunting.

7mm-08, 139 SST/[email protected] bc, 2800 fps., 300 feet elev., 85 deg., 29 bar. pressure, 80% humidity

Range	Velocity	Energy	Trajectory	Come Up (MOA)	Come Up (MILS)	Wind Drift	Wind Drift (MOA)	Wind Drift (MILS)
0	2800	2420.0	-1.5	0.0	0.0	0	0	0
100	2628	2132.0	1.1	-1.1	-0.3	0	0	0
200	2463	1872.0	-1.3	0.6	0.2	0	0	0
300	2304	1638.0	-9.4	3.0	0.9	0	0	0
400	2150	1427.0	-24.2	5.8	1.7	0	0	0
500	2003	1238.0	-46.4	8.9	2.6	0	0	0

Same load at 9000 feet, 21.5 bar. pressure, 45 deg. and 50% humidity

Range	Velocity	Energy	Trajectory	Come Up (MOA)	Come Up (MILS)	Wind Drift	Wind Drift (MOA)	Wind Drift (MILS)
0	2800	2420.0	-1.5	0.0	0.0	0	0	0
100	2690	2234.0	1.1	-1.0	-0.3	0	0	0
200	2584	2060.0	-1.2	0.6	0.2	0	0	0
300	2480	1897.0	-8.7	2.8	0.8	0	0	0
400	2378	1745.0	-21.8	5.2	1.5	0	0	0
500	2278	1602.0	-41.1	7.8	2.3	0	0	0

That amounts to approx. 2.4" difference in point of impact at 400 yards which is about what I saw last year at the range in Colorado. The increase in velocity and energy is pretty astounding to me. I'll take it!

 

[Newtosavage]

So I'll adjust my zero from 170 yards to 130 yards, and that should bring my reticle back where I like it. Easy peasy.

Range	Velocity	Energy	Trajectory	Come Up (MOA)	Come Up (MILS)	Wind Drift	Wind Drift (MOA)	Wind Drift (MILS)
0	2800	2420.0	-1.5	0.0	0.0	0	0	0
100	2690	2234.0	0.4	-0.3	-0.1	0	0	0
200	2584	2060.0	-2.6	1.2	0.4	0	0	0
300	2480	1897.0	-10.7	3.4	1.0	0	0	0
400	2378	1745.0	-24.6	5.9	1.7	0	0	0
500	2278	1602.0	-44.5	8.5	2.5	0	0	0

 

[prm]

In JBM you can just enter 29.92 for the pressure, then check the ‘std. atmosphere at altitude’ box and it will adjust for you. That’s the standard atmosphere of course vice an actual current measurement, but will get you in the ballpark. If you have the actual pressure, temp and humidity you can enter those and only check the ‘pressure is corrected’ box.

 

[zog]

Wow good answers - I've had the same question. Very helpful, thanks