Advanced Marksmanship Atmospheric Pressure Differences

[Luke]

Is there any quick and easy way to determine POI effects by what the pressure is? If I look at my Suunto today and it tells me 28.30hg is there any way to determine what my dope will be tomorrow when it reads 27.30hg?
Obviously nothing beats gathering actual dope in all conditions but just curious if there is another method.

 

[wmlaw]

Re: Atmospheric Pressure Differences

My data cards are calculated for 29.53 in/Hg. I start my corrections at +/- 2 % for every +/- 1.0 in/Hg change in barometric pressure when shooting .308 match grade ammo based on a 100 yard zero. This will effect both elevation and wind data. With lower barometric pressure your bullet impact will be high and you will need to subtract 2 % from your elevation and wind turret calculations.

An increase in Temperature, Altitude and Humidity will also decrease drag and result in a higher bullet impact as well.

You will need to get a kestrel and a log book and start collecting data. The above is just a starting point. If you are looking for help with all of this, I found Disciplined Precision written by Aaron at Ballistic Technologies to be well written and concise.

 

[Sig685]

Re: Atmospheric Pressure Differences

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Luke</div><div class="ubbcode-body">...is there any way to determine what my dope will be tomorrow when it <span style="font-weight: bold">reads 27.30hg</span>?...</div></div>

You're going to have to put in a lot of windage because at that pressure, you are in a hurricane.

But seriously. The one inch difference of hg that you use as an example is equivalent to about 1000 feet of altitude. So I don't know that it's going to make much of a difference. In my neck of the woods, South Texas, I pay closer attention to the temperature as I find that has some influence on the trajectory at long range.

 

[Lindy]

Re: Atmospheric Pressure Differences

That's what ballistic programs are for. If you don't have access to one, use this one:

JBM Ballistics

But whether you need to do that depends on what distance you're shooting at. Inside 600 yards, you can ignore the difference.

As another noted, a drop in BP of an inch is like going up 1000 feet in altitude. With BH 175 in my .308, at 1000 yards, my dope will change about 0.2 mils or about 3/4 of a MOA.