Barometric pressure and altitude

[grayfox]

So, here I am in Jackson Wyoming doing some note taking in preps for next month in CO. Jackson is elevation 6200 so std atmos pressure for it should be about 24 in Hg.... however, so far for this trip (2days so far) it has been 30.4 - 30.1. Today 30.2 and falling. So this seems to mean air is just as dense as 30.1 in Northern VA, or? So shooting "at higher altitude" is a bit more complicated than just std altitude/air pressure numbers...?

[LRRPF52]

Those are stations pressures corrected to sea level for meteorologists.

Look at your Kestrel for the actual barometric pressure, not the reported station pressures of a given city/airfield/weather station.

I always look at my Kestrel and plug in that value, as well as temperature.

http://www.challengers101.com/AltSettings.html

[Texas]

Barometric pressure is subject not only to altitude, but also weather conditions. At every airport, one of the tasks of an air traffic controller is to provide current barometric pressure to a pilot on take-off and landing.

[grayfox]

Those are stations pressures corrected to sea level for meteorologists.

Look at your Kestrel for the actual barometric pressure, not the reported station pressures of a given city/airfield/weather station.

I always look at my Kestrel and plug in that value, as well as temperature.

http://www.challengers101.com/AltSettings.html

I don't have kestrel, I have an app called altimeter v.2.0 on my android. But this starts to make some sense, in the app display there is the analog dial altitude "clock" and then 23.94/ 6217/ 30.08 displaying at the bottom. Appears to be actual pressure, altitude, then a sea level value...?
I'm reading the links online now that you all posted.
Once again I learn something new!!!

[LRRPF52]

Yes, 23.94" of Hg is very typical for 6217 feet elevation.

If you don't have a Kestrel or watch with barometer, you can use a graph with different pressure values for different altitudes as a general guideline, but these will shift with weather systems that come through. Low and high pressure weather systems have different atmospheric effects to them, and will change your values, so this is why I like to have the Kestrel so I can enter the exact values for barometric pressure and temperature.

Humidity is often talked about, but has almost no effect out to 1000yds. If you enter in your humidity values with 2 different samples in your program, one starting at 0% and the other at 100%, look at how much it affects the bullet at 1000yds. You never have shifts that drastic, so most of us just input 50% humidity and don't look at it again.

If you're shooting ELR well past 1000yds, now you want to enter in the humidity when looking for as close as possible of a 1st-round connect, which becomes less and less probable once you start getting out past 1200yds with a more ELR-capable cartridge. A lot has changed for hit probability though due to the programs and data we now have, as opposed to the days of log books and guessing.

[grayfox]

Thanks 52!
I'm basically figuring to use the altimeter app on my Samsung... I'll be calibrating it once/day at the camp's known altitude...
it's also got a gps app so even in airplane mode (to save battery life) I've got a topo map onboard..... along with my paper one of course!
But as for kestrel, I just did a search and there are several models. So I'll look 'em over!

ps, I do have a temp/wind speed meter, too. And making field cards for my various loads for the altitude. This combo is my current plan.

[LRRPF52]

A lot of competitors shooting field matches that involve hiking are using Density Altitude cards.

You can also do this with Dope Disks in Vortex's new scope cover/ballistic data combo product line.

[Sticks]

Dope disks..if you can read them.

grayfox - if you don't have a range finder, this is a smokin deal. Otherwise the simple Kestral 5700 with applied ballistics (sportsman's model) would do you well for $300 less.

I need to get some reliable dope cards made, but for now, after 3 LR matches, I use my Kestral throughout the day to jot down my elevation and windage at each stage. I will take 3-4 temp and pressure readings throughout the day since we are starting at 7:30a and a lot of changes happen through the peak of the day. It may only be .1 or .2 mil, but I need every little bit of edge I can get right now, especially past 600y.

[grayfox]

Thanks Sticks...
Woa! 600 yd, right now that's a bit beyond my league. Looks like some really nice functions, might have to add one of these to my Christmas list. Doing a long range match is just a faint speck on the horizon right now. Once I can settle in at 300 yd then 400-600 would be my next phase.

[LRRPF52]

Sea level, 59˚F, 29.53" Hg, 2500fps, 123gr, .5 G1 BC
300 2010 1103 -17.3"
400 1860 945 -38.2"

Same conditions, up the temp to 90˚F:
300 2040 1137 -17"
400 1899 985 -37.5"

4000 ft ASL, 27.4" Hg, 90˚F:
300 2125 1233 -16.3"
400 2008 1101 -35.6"

Your trajectory is still within an inch at 300yds, and ~2" at 400yds even with drastic temperature and elevation changes. As the distance gets farther, barometric pressure and temperature become much more pronounced.

What bullet are you shooting, and do you have a ballistics program?

[grayfox]

I do, currently I'm using 3 different ones (ballistics), using them to sort of cross check each other since that's all I can do for now, down here.
Plus 1 is on PC, 1 is on my ipad... neither goes onto the other. But that's ok for now.
One thing I've come to conclusion-wise is, since the hunt is pretty close and I've been practicing with these since the Spring, I'm going to try to stick with them since it's what I know.
2 rifles going, main is 6.5 Creedmoor, 2d is 6.5 Grendel AR. All of these loads I have chrono'ed and they have been consistent for the shooting conditions.
For the CM: main load is 129 ABLR at 2597 ft/s. Backup load is the factory 143 Eldx, 2610 ft/s.
For the Grendel: 123 factory/handload (cfe) both are 2510 ft/s. 2d choice is handload 129 sst at 2415 ft/s.

Ballistics programs: PC has PointBlank (freebee but seems to work- I've used it for quite a while), which is able to correct for altitude and temp.
Ipad has strelok (the plain one), which again can adjust for altitude and temps. I won't be taking either PC/ipad out into the field. I probably will have them back at the cabin.
Both PC and ipad can use Nikon's Spoton when the web is available, and I've been referencing it as well.

The strelok has reticle pics at the various magnifications... I have Nikons on both rifles. the 4.5-14 bdc (older buckmasters) is on the Grendel and a prostaff 5, 4.5-18 on the CM. My max planned mag for either one: I plan to go no more than 12x at the farthest distances I plan to shoot.
My eyes are getting "up there" but can see clearly with both of these scopes and the bdc reticle is a common picture for both of them.

The PB, Strelok and SpotOn have all be in agreement pretty much (although exact agreement is not to be expected anyway), but typically at 300 yd, the differences are in the 0.2-0.4 inch range for drop numbers.

For now I figure I can go up to 325, maybe 350 with the CM, and 275 with the Grendel.
For the CM, the Strelok doesn't have the exact scope of Prostaff 5 but I have been able to get a reasonable substitute with a Monarch and adjusting the min-max magnifications; the values correlate to the other two programs up to 400 before they diverge too much; but I'm not going to 400 anyway, so I think it is still a useable setup.
The big thing the Strelok is giving me (besides some cross-checking of PB and SpotOn), is I can make some smallish sight-pix of the reticles with an elk, the ranges and the bdc circles in it, and range diffs on that pic too (for hunting elevation). I'll carry those small pix with me. Picture is worth 1000 words for me!!! But I also will have 25- or 50- yd dope values.

According to the shooting I've done so far, and the drop predictions of the various loads, each of the 2 loads per rifle are hitting very close as to POI's, so that cuts down on the dope-requirements.

You'll also notice that I've got 1 factory load and one handload for each, so I can fall back on the factories if the HL's don't shoot as planned for whatever reason.

I hope this looks like a good plan but if you've got any suggestions I'd of course throw them in the mix.

I also will be verifying zero's and POI's at altitude b/c we'll get there a few days early.

ps, altitudes will be somewhere north of 8800 ft.

[Sticks]

JBM Ballistics is another one that you can plug in your numbers, including corrected station pressure and get fairly accurate results. Copy and paste the results into a spreadsheet.