Range Report Adjust enviro. conditions for ballistics calculat?

[Garvey]

I calculated an average MV of 2740 fps for a .308 load. I entered all of the environmental conditions for the day that I was doing the load testing. The temperature was 93F. I will be shooting that load in Texas temperatures from 30-110 degrees F. My thought was to enter a mean temperature of 70 degrees to build a base range card for the load and rifle. I just entered a temperature of 30F, 70F, and 100F and the JDM calculator did not change bullet drop. I understand that I need to fire the load at various ranges to find the actual drop.

My question is will the MV vary enough that I should chrono the load in as many different temperatures as possible? Will the MV vary enough to cause exteme variances, and thereby not allowing me to trust my mean temperature input?

I don't the ability to plug the numbers in at the range, no internet access. Maybe I should purchase a ballistics calculator to load onto my computer?

Do you see what I'm saying?

 

[azimutha]

Re: Adjust enviro. conditions for ballistics calculat?

Depending on the powder used in your load, it may be temperature sensitive, which would certainly cause some variation in muzzle velocity. As far as external ballistics are concerned, however, temperature may or may not be your biggest concern. Large increases or decreases in elevation will affect atmospheric pressure and that can have more effect than temperature in altering trajectories. But often as not there is a trade off between the two. As you increase altitude (lessening air density) temperature tends to decrease (increasing air density).

The short answer is, if you won't have access to a portable field solution and onsite station pressure readings, then averaging things out like you're doing is better than nothing (and often plenty adequate). You already know to field check your dope. Out to medium ranges there's not going to be a huge difference anyway. At 500 yards, the difference between sea level with 100F, 90% humidity and 8000 feet with 30F and 30% humidity is around 3".

 

[Garvey]

Re: Adjust enviro. conditions for ballistics calculat?

Thanks for the input. I don't suspect I will be taking this round and rifle into much different elevation changes from where my load was tested. In my area we are at about 500' above sea level. Where I deer hunt is between 800' and 900.' I have no plans of entering in any rifle matches outside of a 4 hour drive from home. So it seems that I'll be in pretty good shape. I understand what your referring to about elevation increase reducing air density as well as temperature increasing air density. So the temperature swing was my main concern. Apparently it's not a huge factor in this case.

Now when I chronograph loads for my .22-250 at home and then take them to the Texas panhandle, to prarie dog shoot I'll need to input the elevation for where I'm going as well as at home. The elevation where the prarie dogs are is about 3000' higher.

THANKS!

 

[427Cobra]

Re: Adjust enviro. conditions for ballistics calculat?

Get a Kestrel 4000 and use Lindy instructions to make DA cards, the Kestrel 4000 will give the Density Altitude, the cards Lindy showed us how to make will work in Death Valley to Mount Everast, and of course everything in between.

 

[Garvey]

Re: Adjust enviro. conditions for ballistics calculat?

Looks like I shouldn't have bought the basic Kestrel that only measures temp, wind, and wind chill.

 

[427Cobra]

Re: Adjust enviro. conditions for ballistics calculat?

Lindy also showed us how to calculate DA

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/ManualDA/

 

[desertHK]

Re: Adjust enviro. conditions for ballistics calculat?

To get the most accurate results, I would recommend chrono your load at the various temperatures that you want. For most so called "temperature stable", you will be okay in the "reasonable" temperature range. However, once you deviate to either extremes, you are more than likely going to see fluctuation. Remember, even with 50 fps different, at a 1K + range, can translate to a miss. Strongly recommend getting a Kestrel. DA table is a great substitute.