Range Report Density altitude averages, average conditions for generic ballistics table?

[tucansam]

Hi guys.

I have a question about developing ballistics charts. I've been doing some calculations for the past few days based on conditions in my AOR. I may find myself at 2500ft elevation all the way through 6000ft. Depending on the season, it may be 115 degrees F outside, or 5 degrees F. This is the Arizona Sonoran desert and conditions can vary more than 100 degrees between summer and winter, depending on where I am working.

I'm trying to get some "quick come up data" for my issued rifle at work. If I have the time to pull out my ballistics computer and get legit data for my current position, atmospherics, etc., I will be doing it. But, more likely, I would be needing to get a shot off as quickly as possible, and under exigent circumstances. In that case, making a calculation or looking at Shooter on my phone is going to take too much time.

I've been looking at almanacs and have established average temperatures for the six different areas I work in, depending on month and time of day. I also have put together a list of the areas I usually work, and their average elevations. If I crunch all of the numbers, my 12-month average temp is 65-75 F and my average elevation is 3900ft.

But I ran the numbers in shooter, and then pulled my current data for my current position (which included density altitude). The difference to 300-400Y is a matter of 3-4 inches. But the difference at 600-700Y is 12-25" depending. So at that distance, that's a swing and a miss.

So I'm trying to come up with accurate, but average, data to input into Shooter, that would allow me to create a "reasonably accurate" ballistics chart to keep with me in the field. Something I could refer to if I didn't have time to pull out the other tools.

I'm planning on making several laminated cards to keep with me, based on location (elevation) and temperature (time of year), but density altitude is going to vary, and it seems like the combination of conditions affects external ballistics a great deal.

So how do the professionals do it? Data for each potential AOR/elevation? Average or specific temperatures? How do I calculate average density altitudes? Can I find average dew points and average baro pressures for my areas too? So far I've only been able to find average temps.

Advice welcome.

 

[Lukas Koukal]

Average will always work... around those middle conditions only. Depends on the distance you expect to shoot at, too - the further you go, more exact calculations are needed.

I use DA tables myself, and I follow Lindy´s step-by-step (thanks again to the author for having introduced me to the subject):

Making Ballistic Cards Using Density Altitude
Manual Calculation of Density Altitude

A table out to 1000m in 50m increments and in Density Altitudes from -3000 to 7000 in 500ft increments fits onto a single A4 page (with JBM you´ll need to make and join 2 tables as it only allows so many DA inputs at once). I then add a conversion table like this:

http://www.cfinotebook.net/graphics/performance/Density-Altitude.jpg

and always have an outdoor watch by the rifle.

The procedure in competition when time´s critical then goes:

1. Check the temp and pressure on the watch;
2. Find the closest DA in the conversion table;
3. Find the solution in the DA table.

...takes seconds, really. Enough for 1st round hits on steel if I do my job as the shooter. For further distances (where any error is more pronounced) or much smaller targets, I find a computer necessary. But for silhouettes up to 1k, this piece of paper and a wristwatch work remarkably well when speed counts.

 

[Deleted member 10043]

I got my averages from historic data.

http://www.wrcc.dri.edu/cgi-bin/clilcd.pl?nm23009

 

[tucansam]

Thanks. Here's another chart I found (with temps in F as well as C)

http://www.asiresource.com/DensityAltitudeChart.PDF

Only question is, how do I use the chart? Do I need pressure altitude first? If I have temp, my elevation above sea level, and baro pressure, what's the next step?

 

[Deleted member 10043]

You don't need elevation. You just need the recorded average station pressure, temp, and humidity. For example, weather report pressure right now in Roswell is 29.88 but station pressure is 26.31. Elevation would be zero in my app based on station pressure. Average historic station pressure in Roswell is 26.2. Based on historic data I keep the default settings at 26.2, 61 degrees, 48% humidity, 0 elevation. Based on what you posted you might consider a field sniper zero and learn to hold your reticle rather than depend on the knobs entirely. This would be based on your knowledge on historic and current information as well as experience.