Barometric pressure, also know as atmospheric pressure, is another aspect we need to calculate for. This is the pressure exerted by the weight of the atmosphere, which at sea level has a mean value of 101,325 pascals or pa (roughly 14.6959 pounds per square inch) which is equivalent to 760 mm Hg, or 29.9212 inches Hg.
I'll start with ease, then we'll dive deep into the calculation. Depending on caliber and bullet (this is not the most accurate method), a general rule for adjusting for barometric pressure is .1 mil per 1 inHg out to 1000 yards. Also, generally speaking, you will have 1 inHg change per 1000 ft of altitude change. If you are at sea level (29.9 in Hg) and climb 1000 feet, you can estimate that your barometric pressure is now 28.9 inHg, and you will need to make a .1 mil correction (down) from your dope. The same is true if you lower your elevation by 1000 feet; you will make a .1 mil correction up. The further out past 1000 yards, the greater your adjustment will be. The good news is, barometric pressure adjustment is very linear. For example, at 2000 yards, I have 1 full mil of adjustment per 1 in Hg. So, if my elevation changes by 2000 feet, I just dial up 2 mils. This means I can use 1 set of tables and make the correction. I don't need multiple tables for every 1 inHg just to adjust for altitude change. Keep in mind the higher the altitude, the thinner the air is. This means the bullet will have less resistance, and reach its target faster, and this equates to less drop.
Ballistic calculator, Tables and D.O.P.E
The easiest way to make these calculations is a ballistic calculator. Punch in your elevation or turn on your location and let it do the rest. Well, that was easy. Except, I think the batteries died and, of course, I forgot to bring extras. Or, it's pouring down rain, and I can't keep the phone dry enough to use the ballistic app. Also, you could be shooting in the desert and it's so bright you can't even see the screen. Not that it really matters anyway, because It's 104 F°, and the screen is turning white from the phone overheating. This leaves us with our drop tables and D.O.P.E book.
Your D.O.P.E. book should have all of the information in it from previous engagements, and that will be your most valuable tool. If you have logged the information correctly, you will know exactly what your rifle and bullet will do in matching conditions. Our dope books are used like drop tables, but are confirmed to exact atmospheric conditions. We can use the information we have collected to build a drop table. The more information you collect, the easier building your tables become. Think of it like a sudoku puzzle.
Just to be as honest as I can, I don't have very much personal experience with changes in barometric pressure. The difference between 2 of the ranges I shoot at are only about 600 ft in elevation. That's about .5 inHg. Theres not a huge change in POI. At 1400 yards it's only .2 mils change. But, .2 mils at 1400 yards is about a 10 inch change in POI. So this is how I build my quick reference card for barometric change.
Using a ballistic calculator (there are links on this website for free ballistic calculators),
insert all the parameters with the correct information from your D.O.P.E book or collected data. Make a drop table of your dope out to your maximum range. Next, go to the pressure input and change it by 1 inHg. The difference between the two drop tables is how much dope you need to add or subtract per 100 yards for every 1 inHg change.
Wrench In The Spokes
We know the rate that our bullet falls doesn't change due to environmental factors because gravity doesn't change. But instead, with a decrease in barometric pressure, the air is less dense and means the bullet flys through the air with less resistance. The effect of this is the bullet is exposed to gravity for a less amount of time and will result in a higher POI if no correction is made. Awesome, pretty simple right? Except, we also know colder air is more dense than warm air. And we can typically find cooler air at higher altitudes. Welcome to the complexity of long range shooting.
The Overwatch
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