Over-thinking barometric pressure...

[Dan Newberry]

In this age of gadgetry we're seeing more and more emphasis on barometric pressure (or density altitude) calculations when figuring your bullet's trajectory. Do these numbers really affect your bullet's flight? Yes--of course they do. But the question is, how much?

Our long range shooting training curriculum places a strong emphasis on freeing ourselves as much as possible from dependence on anything that requires a battery. It is our belief that if you become too dependent on your smart phone or your Kestral--or even your range finder, you're going to be rendered largely ineffective in a situation where you don't have these amenities. (And we must now wonder how anyone ever worked without these things, too, right?)...

How does one get along without a range finder? Ranging targets in the real world with a reticle is extremely inexact (you won't have nifty little 18" squares to mil anywhere except on a rifle range, most likely). But you can make range cards using that LRF when it is working, and light conditions are favorable to good readings. And failing that, working in the "off season" you guesstimate the range, make a shot, note the impact, correct the scope setting, hit the target, then record that info on the range card. I'm digressing here, I know... but I figured the question would come up about LRF's so I brought that out...

Barometric pressure can only be known though the use of electronic devices (and you'd be surprised how these things can give different readings from the SAME place sometimes). You can go to Weather Bug on your phone, and it'll tell you... if you have Kestral, it'll tell you... but if electrons cease to flow through your wires, you're out of luck. How bad might that end up being?

Actually, not as bad as many would lead you to think. If you're using a decent long range bullet (and if you're not, you've got no business shooting it past 500 yards), then the average barometric pressure swings in a given area are not going to be enough to move you more than a half minute off vertical at 1000 yards. I shoot mostly the 178 Hornady AMAX these days, at about 2620 fps. By looking at the yearly highs and lows for barometric pressure in my area, I can see that the most I'd be high or low (if I base my drop chart on the average) would be about 5 inches at 1000 yards. Heck, I miss by that much anyways most of the time.

Here is a local weather station chart for my area. Note the barometric pressure high and low for the year...

Weather History for Weather Station KVAWYTHE1 | Weather Underground

If you're using a bullet that has a higher BC than my .308 bullet does, and especially if you're driving it faster, you'll see even less of an issue.

NOTE: I'm not saying that you should disregard your barometric pressure input (or density altitude adjustment) if you have access to that information. I'm simply saying that you ought to make yourself aware of what to expect in your given area as far as pressure swings throughout the year.

The big things affecting your trajectory are altitude and temperature. If you're keen on these two numbers, you should be able to hit within 1 MOA high or low at 1000 yards with any decent long range load recipe. And temperature and altitude are two things that are not hard to know without your smart phone or your Kestral or whatever.

Dan

 

[CavReconScout]

This might be a dumb question, as I am just figuring this stuff out, but isn't this the thinking behind the FDAC? You use your altitude and temp to figure out your DA?

 

[Dan Newberry]

Here is an aviation article on the matter... but you still need a functioning pressure gauge of some sort...

Calculating Density Altitude with a Pencil | Flying Magazine

 

[nuclear_shooter]

I'd love a good read on the actual physics and math behind these ballistic corrections and calculations.

 

[Dan Newberry]

I'd love a good read on the actual physics and math behind these ballistic corrections and calculations.

The good news is that such material is all over the 'net... the bad news is, it's often confusing and very contradictory.

But more good news is that we don't generally need to concern ourselves with the "eggheadery" that so often worms its way into a craft that needn't be so complicated. If we major on the majors, and avoid getting wrapped around the axle of lesser matters, we'll still hit our targets just like riflemen have been doing for generations...

Dan

 

[Lowlight]

The anti-tech movements always makes me chuckle, while it sounds great on paper, it's completely misplaced in my opinion.

I look at it the same way I do with my fire extinguishers, sure I have 3 in my house, and I have determined the best place for those 3 are, 1 in the garage, one by the furnace, and the last one in the closet between the bedrooms. I took the time by buy them, set them up for use, but after that, it's "break glass in case of emergency only".

The best way to address no-tech is with tech... the old school formulas are completely outdated and not nearly as accuracy as people from 1978 claim. With modern computers, a printer and lamination paper you can set up some charts specifically for your rifle and load combination in much less time and with much greater accuracy then trying to go old school. That is a learned process.

Regarding barometric pressure, it's much more a controlling factor then you allude too. Altitude is just a just a factor of barometric pressure, which is why with absolute pressure you can ignore altitude. When you use the actual number and not corrected pressure the software gray's it out so you can't use it.

If you are a home field shooter, never changing locations much, sure you can follow the "ignore it" suggestion, however if you travel, it's the deciding factor. Prime example. I shot the Oregon Sniper Challenge in August. My home range is in Colorado. My dope to 800 yards in CO was 6.5 Mils, when I was required to shoot a cold bore shot at 800 yards in Oregon my dope was 7.2 Mils. That is not something you can overlook. Altitude alone was not gonna do that, I used my Kestrel and software.

if you want to go battery free, the FDAC from Adaptive is a self contained solution. You can do everything without any power using Density Altitude. It's the best field expedient way to adjust your shot to hit your target. It has a scale on to help you determine the correct DA without a Kestrel or phone. So why would you NOT use it... Density Altitude is what I use in my Databook. In fact I removed Humidity because it is the one factor we can overlook and I replaced it with DA. (See impact databook page, "Shooter's Diary")

Small light very similar to a mil dot master, it's worth the investment and the time to know how to use it in the field. You have a higher degree of accuracy using this device than something else. As well you can change it to work with big swings in DA. Teach this, old model with a new solution.

Another note, on home field, one hour from my house I have one range I use, the average DA is 7500ft, go one hour in the opposite direction in Pike National (another shooting spot) my Average DA is 10,000ft. I have shot in Pike with my DA at 15,000ft, especially May through Sept. That is a huge swing for me. It's much different than going to the same square range all the time.DA is a quick and easy way to adjust for this. I advocate using DA in the field, it is one number that tell the bullet what it wants to hear. Doesn't get easier than that one number.

I honestly would rather see people promote the proper use of a Kestrel and Ballistic Computer and how to make accurate back ups, then to give them rule of thumb, 1 MOA changes at X Range shortcuts. Here is one example for a No tech wind card I use, for my rifle and system. It's tailored, done on a computer.



Using this much more effective than trying to give someone the formulas and pretending like you'd do the long hand math in the field. I have a similar card for ranging. In my Long Range classes I spend about 15 minutes on formulas, all of the originals. I show the drop and drift formulas, how to convert, and how to get constants for your rifle. But then I spend over an hour showing people what goes into using a computer. The proper way to set them, what needs to be do to true them and I absolutely caveat everything that all computers are just a starting point for establishing your dope. Would it not serve more people to educate them on their use and limitations vs rehashing formulas from the 1960s that don't even address the most common modern bullets?

Sure you can fill your Databook with all kinds of pages, but teaching people to use the new tech is far more effective in my opinion. And honestly, my Kestrel with Applied Ballistics is 10x lighter than my databook. My databook is packed with information. It's thick & heavy. The batteries on the Kestrel last forever, it's super accurate (what I used in Oregon to hit my cold bore dead on) and its really, really small and light. It's a no brainer that if you can only own one device the Kestrel is the one to own. Without your Kestrel your no tech is close to useless, cause it feeds your DOPE / Databook when you are setting up all those no tech solutions. You had to get your initial information from some where. You can't start off by operating in a vacuum.

No, Barometric Pressure is important. Maybe not if you haunt the same range day after day, but if you travel, if you hunt, hike, or move around, it's the best tool in the toolbox. Learn the proper way to use the tools, and make back ups that you put in glass for the emergencies. When you re-enforce your personal library over and over with solid, verified information, no tech is easy, most of that can come from memory. I can recite my dope on an average day without the aid of reference. But that comes from constantly tuning my gadgets and seeing it. I shoot a lot of different loads, rifles, etc, and can roll into one from the other thanks to my PDAs, Ballistic Computers, etc. At K&M last month I took a rifle I never shot, I was handed a bullet I didn't load and never used before, and landed in the Top 15... I chronographed it Friday, went back to my Hotel Room, fired up JBM, and made dope cards using the computer. They handed you COF Books the day before the match. I attached a post it note to each page. When it was my turn to shoot a specific Stage I pulled off my Post It Note and used that information. No gadget came out during the match, but I absolutely used my computer prior to set everything up. Teaching that is of greater value than any rule of thumb that was written for a 173gr back in the day.

 

[Dan Newberry]

Good points, Frank, and I do understand your philosophy.

I do agree that if you're going to travel and shoot, allowances will have to be made to have a prayer of a first round long range hit, and devices such as the Kestrel and your smart phone (and that admittedly nifty FDAC card) would be extremely helpful.

I find that the majority of folks that I work with have some things in common... they probably won't travel; they'll plan on sticking to their general location for hunting and target shooting--and defense of hearth and home God forbid. And most aren't likely to purchase what they would consider expensive units such as a Kestral. Some don't even have smart phones. They don't have the kind of cash that would be needed to put together a custom rifle with a state of the art scope. But many of them end up being very good long range rifle shots, and that's a commodity America is going to need more of if we're to hang on to our freedom in the age to come.

As I mentioned in the original post, I wouldn't dissuade folks from using their electronics, and I do agree with what you've said about making some hard copy, printed back ups for cases when these things fail us, for whatever reason. The FDAC card is a good idea, and appeals to any of us who wonder how well we'd do without a Kestrel and/or smart phone handy. Thanks for sharing your perspective, it's always worth giving heavy consideration to.

Dan

 

[Lowlight]

In my class I use JBM, it's free. They can make a variety of range cards including DA and Wind all for nothing.

The specifics to use JBM works with all other software. We chronograph the rifles in class and then return to the classroom and I demonstrate using the software. Requires nothing more than a web connection. I do it right through my PowerPoint presentation.

 

[Dan Newberry]

In my class I use JBM, it's free. They can make a variety of range cards including DA and Wind all for nothing.

The specifics to use JBM works with all other software. We chronograph the rifles in class and then return to the classroom and I demonstrate using the software. Requires nothing more than a web connection. I do it right through my PowerPoint presentation.

I use JBM too... it's a useful tool for sure. It seems like it's more accurate than a lot of the programs that we have to pay for.

 

[mouse07410]

Great discussion!

Oh, and Dan - the information your Web site provides is priceless, but that Adobe Flash format sucks (IMHO). I'd love to see it offered as a PDF file, and could help with that if you wish (just PM me if so). I assume you want people to read it, print it, share with others...?

 

[Dan Newberry]

Mouse, yes... that would be good. I'm not an IT guy at all... I cobbled that site together years ago with some DIY software. And I guess it shows...

And in the always learning something new category... I had not taken a very serious look at the FDAC card that Frank showed. Obviously, they've figured a way to estimate DA by using only the altitude and known temperature. That's the kind of thing that I really like, something useful and relatively simply--and no batteries.

There must be some formula that we could use in a given area to tweak the trajectory with just a bit more precision than the traditional coarse adjustments, i.e. 1 MOA @ 1000 yards per 20 degree temperature shift etc...

 

[Dan Newberry]

a good read... and helpful along the lines I mentioned above...

Manual Calculation of Density Altitude

 

[kraigWY]

I suppose it doesn't hurt to plug in altitude 'n such. But I don't normally worry about it.

I live at 4500 ft. I just got back from elk hunting where I shot my elk at 340 yards (GPS said 342, LRF said 344) at 8900 feet. I forgot taking into account altitude difference. After reading this topic I checked my Iphone shooter program. I should have made a "1" 1/4 in click change. At 340 that would mean a .85 inch difference. The heart/lung area on the average elk is about 15 inches.

For practical hunting distances, DOES IT REALLY MATER?

At my age, w/COPD, climbing around mountains at altitude, a good steady shooting position is more critical.

Yes things change over the years. In the last 40 years, mountains have gotten steeper, and there is less air at altitude now. I did find that less I carry, the better off I am.

 

[mouse07410]

Dan, please check your email (I used your @bangsteel.com email that your Web site provided), and tell me whether you like it.

TNX!

 

[Dan Newberry]

Looks great... I responded in the email. Thanks for taking the time to do that.

@ kraig... I see your points. You're kinda like me, your shooting is confined within a general geographic location where large adjustments for density altitude are rarely necessary. This is how 95% + of long range shooters are going to operate, I believe.

For someone who travels and shoots nationwide (as does Frank) adjusting for altitude is not an OCD thing--it's a necessity, for sure.

I'm thinking there's gotta be a way to generate a printable chart, perhaps a regional chart, with the minor corrections that may be needed when the temperatures change. Something "poor man's" version of the FDAC.

 

[Pinecone]

The point Dan was making was that PRESSURE is not a huge factor. ALTITUDE is.

Yes, while density altitude takes into account pressure altitude, the difference if you use actual altitude is not huge.

Looking at Dan's link at his location, over this year, the pressure has ranged from 29.70 - 30.40 inches of mercury.

Using the pressure altitude calculation from his link to Flying magazine, you looking at a variation from 220 feet low to 480 high from just using the actual altitude.

So maximum error is 480 feet pressure altitude from his 2500 actual altitude.

Taking my .308 load that I shot at Dan's course, at 29.92 pressure (75 degrees 80% RH), the come up for 1000 yards is 10.6 mils. At a pressure of 30.40, the come up is 10.7 mils, or miss the center of the target by 3.6 inches. Hmmmmmm.

Bottom line, you can ignore pressure without a huge error, but you MUST make adjustments for altitude and temperature.

Take the same conditions above, but raise the temp by 15 degrees, that 10.6 mils becomes 10.4 mils.

 

[Lowlight]

Altitude is a function of barometric pressure when calculating air density, so we only use altitude to change the barometric pressure.

So what you need to do is ignore Corrected Barometric Pressure & Altitude and just use Absolute Pressure with no altitude.

You guys have it backwards, it's the air pressure that matters and not the altitude.
Here is a clue for you, density altitude has nothing to do with altitude, it uses barometric pressure, temperature and humidity. The shortcuts use altitude only. DA is a function of air pressure, not altitude.

At Rifles Only they are at 78ft above sea level, yet the DA can easily be 3000 or even -2000 depending on pressure and temp.

 

[Whiskey 2-1]

DA can be valuable no matter your style of shooting. Like many other things we train for, I may not be useful everytime, or even often, but when you need it, it may be all the difference.

Here is an example of why paying attention to DA on the same range can be important:

I have seen 2000+ swings in DA in a single day at the same range. This can result in a complete miss at 1K. This has happened between the first and second days of matches at Rifles Only more than once. The vast majority of people were missing low on the thousand yard cold bore. Those who noted the change and either checked a laminated card or their program were in a much better position to get the 100 pts.

Another example, a southern boy like me traveling for northern or Rocky Mountain matches/ long range elk hunting on public land.

As Frank said, I have found DA to be a great single number to keep in my head (and data book) to base my elevation adjustments on.

YMMV
Sean

 

[Dan Newberry]

I wouldn't say we have it backwards... just going about it another way.

If you did use an altitude input of 75 feet above sea level, at a temperature of around 50 degrees you'd have something very close to the 75 foot ASL adjustment you'd use on your chart. If temperature would rise toward 90 degrees, you'd be using a 2000 foot input... and if it got on towards 100 degrees, you'd use closer to 3000 feet. With my own load, that would result in a 2.5 minute variation at 1000 yards. And this would be what the FDAC card would allow you to do, but I think with some physical and/or mental notes, a man could dial the shot pretty close. That's all I'm talking about here--just a way to work around the necessity of a precise barometric pressure reading, for instances if/when doing such might be necessary.

If one were to simply use the normal pressure reading for that area, constructing the chart for a temperature of 50 degrees F, then adjust the chart as temperature climbed, you could get the data pretty close--I would say close enough to make good hits on practical sized targets.

It's true that with a Kestrel or other actual pressure data, you would get a more accurate sight setting. But what I wanted to bring out, and what I believe is worth teaching, is that you can construct a simple chart which will allow you to tweak your ballistic card for temperature changes, even if you cannot (for whatever reason) know your actual pressure.

I don't think there's a discrepancy here--as I'm not asserting that your shot wouldn't perhaps be more accurate with a true and precise pressure reading. I just wanted to point out that if one did have to work without true pressure readings, it's still possible to make a reasonably accurate shot with nothing more than a temperature adjusted sight setting.

Dan

 

[Dan Newberry]

MD Shooter... good signature line. Me too...

I'm not denying that density altitude is important, and of course whether we call it density altitude, or simply correcting for temp swings, we're going in the same direction. The man with the Kestrel would get a much more accurate sight setting, but the guy "winging it" could also get onto a 2 minute plate at 1000 yards with a mere adjustment based on the temperature change--the change that actually drove the DA change in the first place, of course. I hope this is making sense...

Dan

 

[nuclear_shooter]

The good news is that such material is all over the 'net... the bad news is, it's often confusing and very contradictory.

But more good news is that we don't generally need to concern ourselves with the "eggheadery" that so often worms its way into a craft that needn't be so complicated. If we major on the majors, and avoid getting wrapped around the axle of lesser matters, we'll still hit our targets just like riflemen have been doing for generations...

Dan

Practically speaking, it is totally unnecessary for making shots. But I'm a physicist, so the eggheadery comes with the territory . Its more a matter of personal curiosity... I don't plan on pouring over equations trying to calculating my firing solution by hand. When it comes to the field, its always better to keep it simple.

 

[Lowlight]

It sort of demonstrates why DA is the best single number to use in the field.

We we start off saying, skip barometric pressure, look at altitude, then it quickly goes to temperature.

So, knowing we can use an FDAC and DA without any electronics, it makes sense to condition your thoughts towards using DA.

You ought to know your altitude roughly, you can wing your temp within 10 degrees, so using the FDAC to get your adjustment is a great single stop solution.

For or me, I know my watch works well to get my basic data, either my Suunto to Casio Pathfinder give accurate data that can be used. If your using electronics, then it makes sense to skip altitude and use absolute pressure. My Casio uses no batteries, is always with me and works great for temp, barometric pressure and altitude.

 

[Ring]

here is the good news....

as long as you dont shoot past 400y, weather has ZERO effect on your shots... DA 0ft to DA 10000ft... nada..... zilch...

past that, do what LL said... print shit out...

 

[Dan Newberry]

Ring... that's very true. But of course we are wanting sight settings for longer ranges, too.

I think we're pretty much all on the same page here, especially after watching Frank's review of the FDAC inserted in this article here--> GEAR REVIEW: Field Density Altitude Compensator « Daily Bulletin

...and the comments posted after the article are good too.

For you guys that own the FDAC card, do you find that you're using it in lieu of your ballistics program? Or simply as a back up? Without being able to true the pre-formatted trajectory, it would have to be off a little here or there--what with velocity variations, scope height, etc. But I do still think it's a great idea.

The Michael Field Density Altitude chart is a handy thing... I think it's well worth keeping a copy of... I drew a red line at 2500 feet to show the DA at the various temperatures in my area.


With this little graph, you can use the trajectory that you have trued for your rifle, and of course you can print off other ballistic cards to match varying DA's that you might be shooting in.

There will be less precision here, than if you consult with a known trustworthy barometric pressure indicator, but a general idea of temperature and a close guess at density altitude will get you on target in cases where your electronics are not available.

Based on a careful perusing of all our posts so far in this thread, I think we've said essentially the same thing, in slightly different ways. If I've been misunderstood here, I'll humbly take the blame for that.

Dan

 

[Papa Zero Three]

For you guys that own the FDAC card, do you find that you're using it in lieu of your ballistics program? Or simply as a back up?

Quiet honestly, it's as accurate as most ballistic programs. By that I mean it might have a varience of +/- .1 from what the PDA spits out as a firing solution. When I dont want to fire up the PDA I grab it. IMO, it works perfect for what it is designed to do. I'm surprised more people don't use it as it doesn't need trueing,it's simple to use and understand, it just works as long as you're in the ballpark with the temp and Altitude and you have the right card in it for your load. I do carry it as a back up to my PDA however but I'm just as confident in it as I am the PDA.

 

[Ring]

also, its vary dependent on where you shoot... if your in FL, your DA wont be changing much, +-1000 maybe...

im in ohio, 90% of the state is 1000ft ASL, my DA is 800 to 3000. but the summer average is 1500-2000

if you live in CO you may have a bigger issue...

 

[Rifles Only]

FDAC all the way. The only issue I have had with it ws summer in the mountains when the DA was above 12k. Then I followed the trend FDAC had from 8k-10k. Easy to do with very little variance

 

[Dan Newberry]

FDAC all the way. The only issue I have had with it ws summer in the mountains when the DA was above 12k. Then I followed the trend FDAC had from 8k-10k. Easy to do with very little variance

I'm reviewing the FDAC right now, courtesy of Mark Williams of Raven Rifles who had one on hand to loan me. Numbers do seem to work.

I had the honor of having Scott Thompson with us last week. Scott is an international airline pilot, and I was able to discuss Density Air with him for a while. He told me that all they ever use to calculate density air is actual altitude and temperature, and that they don't worry about barometric pressure.

This of course does bolster the case for the FDAC card, which I'm seeing is a good thing to have. Running JBM and comparing the FDAC's readings show that it's obviously accurate enough without actual barometric pressure readings.

For folks who are using cartridges which are not compatible with the FDAC card options, some good notes on some drop charts built for the various altitudes you may shoot would work just fine.

Dan

 

[Pinecone]

Scott, is forgetting some things. Airliners do this automagically with computers.

In aviation, to do density altitude, you set the Kollsman window (baro pressure correction) to 29.92 and read off the indicated altitude. That is actually Pressure Altitude. Which is actual altitude corrected for baormetric pressure. So in flying, you DO take into account barometric pressure, but it is not input in the calculation, as you sort of pre-calculate it.

If you use something like a Kestral, you are getting a pressure altitude, as there is no input to put in a different barometric pressure. Same thing with a hiking altimeter. In aviation, these are called non-sensitive altimeters, as they cannot be adjusted for local barometric pressure differences. And when trying to avoid hitting the grond, baro pressure is IMPORTANT.

And with the FDAC, it depends on what you are using for your altitude input.

Only if you use a GPS or map to find your altitude are you not using a barometric pressure correction.

But as the calculations show, if you ignore baro pressure and use only GPS altitude, you are not that far off. If you use altitude from some other source, baro pressure may already be factored in.

As for density altitude not being important in FL, on a cold 40 degree day, the density altitude can be -1191 feet, and on a hot 100 degree day it can be 3186 feet, for a delta of 4377 feet.

Denver over the same temperature range the density altitude goes from 5373 to 9664, or a delta of 4291 feet. So higher elevations have LESS change in density altitude for a given temperature difference. Not much, but some.

 

[GlockandRoll]

As a licensed pilot, I tend to think in terms of density altitude, or what we call pressure altitude.
Great discussion.

 

[nuclear_shooter]

The method I use currently is read the density altitude off my Kestrel (which should take into account pressure/altitude, humidity and temp) and have charts for every DA between -1000 and 8000 in step sizes of 1000. Works very well so far. JBM has a nice calculator for generating these tables: JBM - Calculations - Trajectory Card. If you don't have access to local weather data you can probably get good estimates from nearby weather stations.

 

[30mm]

Since I burn jet fuel for a living and shoot for the enjoyment and challenge, let me offer a perspective that has not been raised in this discussion so far.

The effects of Density Altitude (DA) have the exact opposite effect in aviation and ballistics performance. In aviation, the lower the DA the better the performance of the airplane. The challenge in aviation is to create lift which dense air does very well. Dense air contains more oxygen than less dense air. Cold, dry air at sea level provide the best performance conditions for an aircraft, either jet or piston engine. Dense air into a jet engine creates more efficient thrust for a given input of fuel to the engine. Bullets on the other hand are looking to travel through the air with the least amount of resistance possible. Dense air containing high oxygen content provides resistance to the bullet which requires a higher trajectory to counter (dial more mils). As the air becomes warmer, humidity increases and or altitude increases the density of the air decreases. Warm humid air contains water contains less oxygen that cold dry air lowering the resistance of the bullet through the air.

An altimeter reading is purely a measure of barometric pressure. Weather systems are referred to as a low or high pressure system. High pressure systems are characterized by dry air with good visibility. Low pressure systems contain moisture and lower visibility. High pressure provides good performance for flying, low pressure is good for shooting. The altimeter setting of 29.92 is the atmospheric pressure for a standard day at sea with a temperature of 68 F. It is purely used as a reference. When aircraft fly above 18,000 FT in the US, we all set our altimeters to 29.92 so that each aircraft is using the exact same setting for reference. Since there is no risk of contact with terra firma in the lower 48 above 18,000 feet, there is no risk of hitting the ground using a standard altimeter setting of 29.92. As aircraft descend below 18,000 feet local altimeter settings are used to provide accurate readings. In countries where terrain is higher, the rules are different for that reason.

This may be more information than you are looking for, but there is a simple way to think about it density altitude. If you are at sea level and have no problem breathing the air is full of oxygen - low DA. Climb up to 10-12,000 feet and you notice its much harder to breath because the air contains much less oxygen than at sea level - high DA. Temperature, atmospheric pressure and altitude all affect density altitude because they all affect the amount of oxygen in the air. Your body will perform better at low DA, because it needs oxygen, as opposed to the bullet which meets that oxygen as resistance.

 

[Dan Newberry]

Pinecone, thanks for the comments. I know that you are a pilot as well (can I mention you flew an A-10 tank killer?)... I hope so, as that's something to be quite proud of. And 30mm it's good to hear what you shared as well... your screen name makes me wonder if you have flown A-10's (?)

I guess the question we need to answer is this: Does barometric pressure vary enough to prevent a good shooting solution from being derived from nothing more than altitude and temperature?

If the FDAC card works--as many attest that it does--then the answer to that question must be "no," I would think.

 

[Aircraftmech76]

If you have a barometer ( I bought a Sun analog), you can read your current site pressure. You will lose about 1" per 1K of altitude. The result will be your pressure altitude. Then calculate DA by entering the temp. I use my E6B with the slide removed for a compact way of calculating DA. Nice, lightweight package.

Example: The barometer/altimeter reads 28". Since sea level pressure is 29.92" on a standard day, I have a difference of about 1.9". This means I'm at 1900' Pressure Altitude. Enter the temp in the Pressure Altitude window of the E6B, and read the DA in the same window. Simple is as simple does.

Kevin

 

[30mm]

Pinecone, thanks for the comments. I know that you are a pilot as well (can I mention you flew an A-10 tank killer?)... I hope so, as that's something to be quite proud of. And 30mm it's good to hear what you shared as well... your screen name makes me wonder if you have flown A-10's (?)

I guess the question we need to answer is this: Does barometric pressure vary enough to prevent a good shooting solution from being derived from nothing more than altitude and temperature?

If the FDAC card works--as many attest that it does--then the answer to that question must be "no," I would think.

Dan, yes I also flew A-10's (Desert Shield/Storm '90-91). 23rd TFW "Flying Tigers" - Burma to Baghdad

Barometric pressure usually doesn't change that quickly to make a difference unless a front is moving through the area. Every .01" of barometric pressure is equal to 10' in altitude, so to experience a whole 1000' change in atmospheric pressure you would have to go from 29.92 to 28.92 which is not something you will normally see short of a hurricane. A normal change in altimeter readings throughout a day may be 29.82-30,02 about a 200' difference which is negligible when looking at a 200' difference in DA. Plug different DA's in a ballistic program and look at the results. Like has been previously mentioned in this thread you only begin to see a difference in dope past 5-600 yards as DA changes.

 

[Tburkes]

I've had a set of FDAC cards for a long time and they work great. They also have a compatibility matrix on their site that will help you decide what cards you need for your particular caliber. The guys who run the company have tested the data all over the world in different conditions to confirm the data.

Link: The Field Density Altitude Compensator (FDAC) - ACTS Virginia

 

[Lowlight]

Dan,

There is other calibers available through the Impact Databook pages that are made by ACTS. Adaptive Quick Card (AQC)

The Adaptive quick cards which you can stick right in your datebook. Same information.

Also you can make DA Cards either manually, or with programs like Patagonia Ballistics ColdBore. Which Coldbore is nice because it is one of the only ballistics programs that has a desktop companion as well it links to the mobile device. You can build your rifle / bullet combinations at home, sync to your PDA or just print out your DA Cards for your exact load. Patagonia is pretty inexpensive considering what is out there. Patagonia Ballistics - Features and Integration

We live in interesting times where we have more information and options than ever before. And it just keeps on getting better.

 

[Dan Newberry]

I have a question regarding the FDAC...

They are spaced in 50fps increments for a given caliber/projo.

Obviously if your 10fps under/over the stated card you simply pick which is closer.

But what if your exactly half way in between.

Which card would better suit your needs?

What would be some ways to determine which is better since obviously not everyone shoots in the exact same environments?

Or is it simply a wash?

My situation is 308 175/SMK @ a tested/corrected speed of 2675fps. And the temps can vary as low as -20 degrees and as high 105 degrees with a modest altitude of 1600"-1800'.

I think if you're truly in between 2650 and 2700 fps, you can just choose either card and "pad" the reading a touch past a certain range to compensate for the variation. If you run those numbers across a good ballistic calculator, you'll likely find that there isn't a huge difference in that 25 fps range at most ranges... it would account for +/- 6 inches or so at 1000 yards, depending on which card you chose (high or low).

So you could, say, choose the 2650 fps card and figure on adding .1 mils by 700 yards... and .2 mils by 1000 yards to whatever was called for (or something like that)... not a huge issue, really.

 

[Ryo]

I just wanted to mention that your comment "average barometric pressure swings in a given area are not going to be enough to move you more than a half minute off vertical at 1000 yards" would be incorrect. Depending on the area, there could be a huge swing. On one of my training classes with TBAC, the first day we zero'd the doped our rifles. Based on the collected information from my previous day shooting, I was able to use the app Shooter to plot my firearm's bullet trajectory (muzzle velocity tool on Shooter is awesome!). Any case the next morning I used my Kestrel to get the DA and entered it into Shooter. First round hit at a 600yard target. Guy next to me didn't check his DA (I think he didn't have a Kestrel) and he was swearing up a storm on why he couldn't hit. I told him the DA changed by 2000 and he entered it into his app.. and voila he was back on target.
Note that this was for that area where the temperature swing was pretty big that by noon time the DA had changed enough to effect the trajectory of our rounds.

 

[Dan Newberry]

Ryo... something else is up if you're missing a 600 yard target due to 2k DA change. That is, unless you're shooting at an egg or something such.

 

[Lowlight]

I have seen people miss a Larue Plate at Rifles Only @ 500 yards because they doped the shot at 3PM the day before, at close to 90 degrees and then first thing the next morning it was 55-60 degrees and they missed low. DA only swings about 2k there if that... sometimes closer to 1500ft. Still it can cause an issue when combined with other things like the powder sensitivity.

It depends on several other factors, not just the DA. But I have seen it happen enough to know you can see some big POI shifts under conditions that would be considered small movements in conditions.

 

[Ryo]

Ryo... something else is up if you're missing a 600 yard target due to 2k DA change. That is, unless you're shooting at an egg or something such.

Don't know.. it was this other guy in class. I was on target with the right inputs. There were a few different steel targets, some were 4" disks, while others were 10". Since it was his rifle, his ballistic calculations, his ammo, and different weather conditions (ie wind, DA, etc), I can't say what was his cause of his error, but after he entered the right DA, he started get rounds on target. I'll have to play with the Shooter to see how much off it would be just for my knowledge.

 

[Dan Newberry]

10-4... yes, do check the program and see what it shows. Unless he was using a super low BC bullet, and/or the target was tiny, the DA error alone wouldn't put him off the target.

One thing I do see a lot during our matches is folks will actually either forget to dial the elevation from one stage to the next, or they'll dial it wrong. After a bit of fidgeting they'll sort it out and get on the target. Maybe when you advised him of the DA change, he realized he needed to spin his turret a bit more than a couple of clicks.

Dan

 

[Rifles Only]

We had a DA shift of 3800 last spring from the AM session to the PM session.
We took the time to shoot the conditions and the shift was pretty dramatic at 500 and beyond. Strange day as our usual shifts are not that big.

 

[pinsandpitons]

I don't own a smart phone, tablet, or kestrel. I can read a map and a thermometer though, so coupled with modern computers (to run the equations) DA has been a major positive for me. I'm a relative noob, and DA was a bit of a revelation as I struggled with all the rules of thumb that people tried to get me to follow when I started playing with longer ranges. Maybe I'm too concrete for that system. It's interesting, the availability of high tech solutions has simplified my field firing solutions to a few home-made laminated cards tucked into in my data book. Full circle?

Cool discussion above about DA vs altitude vs pressure. It's these types of threads that add to understanding which I believe to be much more powerful than simple knowledge. I remember reading something from Gunny Hathcock a while ago, something to the effect of pay attention to the details. Where ballistic influences are concerned, many (not all) of those details seem to be well summarized with DA. Pilots have worked with that for many decades, did it take the better bullets and rifles available now for shooters to discover the utility of DA? How long has DA been around in the precision shooting world? Frank?

 

[Dan Newberry]

I don't remember when we first started to hear about density altitude, as far as the phrase goes. But shooters have been using altitude and temperature to account for bullet drop for probably as long as there have been rifles. They just didn't call it "density altitude"... that phrase did come from aviation, and as you point out has made its way into the long range shooting sports. At some point, phraseology becomes so prevalent that folks just pick it up--and if you're going to communicate, you've got to speak the language.

What has probably changed the most is the advent of relatively inexpensive ways of knowing the barometric pressure, and devices (the Kestrel being foremost among them) that will tell you what not only the barometric pressure is, but what the calculated DA is. This is GREAT information to have. Avail yourself of it when you do have it, but have a plan in place to continue working without it, should that ever be necessary for whatever reason.

An aside: Nuclear Shooter may recall that at our long range shoot last Saturday, there were two Kestrels which gave severely different pressures, same place same time... but the one was only unpacked moments earlier, and hadn't gotten it's bearings, apparently. It began trending toward the pressure which was being indicated on the other Kestrel--which was also the same pressure that the Weather Bug app was showing. So it's important to let the units get situated a bit before taking a reading.

Major DA swings are going to be apparent primarily in the temperature. It takes a pretty big temperature swing to move the DA a couple thousand feet. Simply taking note of a large temperature swing will be enough to get you dialing in the right direction.

If I can guess the temperature to within +/- 10 degrees, I should be able to dial a good enough correction to get me on an 18 inch plate at 1000 yards. If I have the equipment to show me exact temperature and DA, the exercise should be to manually discern what I would do if I didn't have the computer. Then check the computer to see how I fared. It's the same thing I do with wind speed calls... estimate it first, then see what the meter says.

Dan

 

[Lowlight]

David Tubb was probably the first one to officially move DA from aviation to shooting. Guys who shot were aware of DA as a lot of them are pilots as well. But David created a system that has since been copied by many. It was David and Brandt Cole that put pen to paper and started moving their Dope to DA.

His system is pretty much all DA based for elevation and then wind MPH for holds. Matching up your Muzzle Velocity to the current DA then adjusting the drop based on changes from that point.

 

[Pinecone]

Pinecone, thanks for the comments. I know that you are a pilot as well (can I mention you flew an A-10 tank killer?)... I hope so, as that's something to be quite proud of. And 30mm it's good to hear what you shared as well... your screen name makes me wonder if you have flown A-10's (?)

I guess the question we need to answer is this: Does barometric pressure vary enough to prevent a good shooting solution from being derived from nothing more than altitude and temperature?

If the FDAC card works--as many attest that it does--then the answer to that question must be "no," I would think.

Yeap, flew the Warthog and shot the gun. Good for tank sized targets out to 2000 yards. From a shooting platform moving at over 350 miles per hour.

I showed the calculations back near the beginning of the thread, but basically, NO, you can ignore baro pressure, but you MUST take into account altitude and temperature.

 

[Pinecone]

Dan, yes I also flew A-10's (Desert Shield/Storm '90-91). 23rd TFW "Flying Tigers" - Burma to Baghdad

Barometric pressure usually doesn't change that quickly to make a difference unless a front is moving through the area. Every .01" of barometric pressure is equal to 10' in altitude, so to experience a whole 1000' change in atmospheric pressure you would have to go from 29.92 to 28.92 which is not something you will normally see short of a hurricane. A normal change in altimeter readings throughout a day may be 29.82-30,02 about a 200' difference which is negligible when looking at a 200' difference in DA. Plug different DA's in a ballistic program and look at the results. Like has been previously mentioned in this thread you only begin to see a difference in dope past 5-600 yards as DA changes.

Hog drivers unite.

I was out by the fun in the sand.

But, I did the numbers early in the thread using Dan's range and baro pressures for the year to date. And the difference at 1000 yards was very small. I just checked, and the come up changed by 0.1 mil from the lowest pressure to the highest pressure at his location.

 

[LastShot300]

The bullet only cares about just one thing : AIR DENSITY period.

And we have 3 ways to obtain it :

1) Baro Pressure + Temp + Altitude + Humidity
2) Station Pressure + Temp + Humidity
3) Density Altitude

In turn we can calculate DA exactly (with the right equations and factoring for Humidity) or approximately.

And what Lowlight said in terms of teaching others, is exactly what I think. Who needs the old, approximate (sometimes plain wrong) formulas, when we can resort to state of the art solutions?

For real ELR work taking shortcuts is a sure invitation to miss the target.

I'll like to add that beware of some programs that compute the AIR DENSITY in wrong ways, and some are very popular unfortunately.