Range Report Effect of density altitude

[farrier]

Looking for a little help understanding how to adjust dope for differences in density altitude. I can't seem to find a formula for this but it must be out there. Example: I shoot a .308 with a 175g. bullet at 2640 fps. I shoot and gather real time dope at a DA of 2500 ft. out to 1000 yds. I go to another range in a different geographical area and my DA is -200 ft. Can I predict accurately the change to allow for first round hits? Is the difference of enough consequence to be concerned with? If so, how do I predict the result?
Thanks, Mike

 

[Lucks]

Re: Effect of density altitude

Most shooters I know use Lindy's method....

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

 

[earthquake]

Re: Effect of density altitude

Look into one of these too...such a great piece of gear!

Field Density Altitude Compensator

 

[Trevor300wsm]

Re: Effect of density altitude

Just running some quick numbers thru FTE, the difference in 2500ft and -200ft DA at 65 degrees is only about .24 Mils at1000yds (with your bullet and MV). Roughly 7.64 Mils to 7.87 Mils.

 

[Lowlight]

Re: Effect of density altitude

Hmm,

When I check the FDAC with a 175gr going at 2650 the is a much more of a difference than .24 mils.

I get 11 mils at 2k even and 10.8 at 3k out to 1000 yards. When I go below 0, to -1k I get round being subsonic and showing 11.6 and moving to 0 it is still subsonic at 11.4 mils.


So the swing is a bit more than .2 mils for almost a 3000k DA change. Almost a full mil in fact, at 1000 yards is quite a bit of a miss.

I would suspect the poster above is doing something wrong with Ballistics.

 

[NOMAD]

Re: Effect of density altitude

If you need a FDAC we have them in stock in the tools section of our website!!

 

[gvanhyning]

Re: Effect of density altitude

I carry a DA chart for calculating DA from know altitude and temperature. The same chart that is used in aviation. I made up a set of DA dope cards at 2K' increments.

 

[9H_Cracka]

Re: Effect of density altitude

DA is an incorrect term. Density is density, period.

Think about that for a bit and maybe it will make more sense.

 

[Trovan]

Re: Effect of density altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: _9H_Cracka</div><div class="ubbcode-body">DA is an incorrect term. Density is density, period.

Think about that for a bit and maybe it will make more sense. </div></div>

"I am trying to think but nothing happens!".
(Curly Howard)

 

[Trevor300wsm]

Re: Effect of density altitude

Frank, or anyone.
I'm certeinly not above making a mistake. I've run those same numbers again, but still don't come up with what you do. Here are a couple screen shots from FTE. Can you see anything I'm putting in wrong?

*****Problem solved. Mixing up G1 and G7 doesnt yield sound results.

 

[Lowlight]

Re: Effect of density altitude

First off your BC is beyond wrong, you have a G1 number with a G7

So your dope is wrong.

 

[Trevor300wsm]

Re: Effect of density altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">First off your BC is beyond wrong, you have a G1 number with a G7

So your dope is wrong. </div></div>

I just spotted the G7 when I was looking back thru it.

 

[Trevor300wsm]

Re: Effect of density altitude

Well hell.....
3yrs of shooting Berger VLD's and using G7 has gave me tunnel vision. I knew that didn't sound right (the original numbers) now that I'm not at work and have time to actually think about it. Hell, that's just a lil more drop than my 6.5's are shooting at 2,900fps. Guess I need to stay off of FTE at work.

 

[Lowlight]

Re: Effect of density altitude

Amazing how that works,