Topic: Sources of Ballistic Program Inaccuracies

[J Mack]

I thought some of you might enjoy this read,

Shooters using ballistic programs often find that their field shooting data does not precisely agree with the output from their program.

In addition, shooters comparing the outputs of different programs often find significant discrepancies between the programs. Why?

Below are listed the cause of some inaccuracies in the elevation predictions of ballistic programs. Windage calculations are beyond the scope of this article.

Uncalibrated scope clicks. For example, Leupold M1 dials I have tested are closer to 1 inch per hundred yards than 1 MOA. With a typical .308 load at 1000 yards, the difference in point of impact is about 20 inches. If you don't know how to check that calibration, you may find this article useful: Optically Checking Rifle Scopes

Normal variations in muzzle velocity. A good load may have a muzzle velocity standard deviation of 15 feet per second. That means that about two-thirds of the shots will fall in the range from 15 fps per second above the average velocity to 15 feet per second below the average. At 1000 yards with a typical .308 load, that range of variations will cause shots distributed over a 10 inch range. Nota bene: About one third of the shots will have variations from the average which are even greater. There are many causes of this velocity variation, and they are beyond the scope of this article.

Temperature variations in muzzle velocity. A pretty good powder will exhibit a variation in muzzle velocity of 1 foot per second per Fahrenheit degree. If you chronographed your load at 85 degrees and are shooting at 55 degrees, your muzzle velocity may be 30 feet per second slower than you think it is. I have measured variations as large as 5 feet per second per Fahrenheit degree. The only way to know how big that variation is, is to test it. In addition, my experience has been that the standard deviation of the muzzle velocity increases at low temperatures.

Muzzle velocity measurement errors. Do you believe your chronograph is accurate? The screen spacing of most chronographs is too short for accurate measurement. Some chronographs like the Oehler 35 series allow extending the spacing. The clock frequency of many chronographs is too slow to get accurate and consistent measurements, i.e., more than one shot with the identical muzzle velocity will be displayed with different measurements. Other common problems with chronograph data is failing to compensate for the distance between the muzzle and the chronograph screens, and failing to fire enough rounds to have a good average velocity. People with little understanding of statistics may have a very vague idea of what the chronograph output means. See Statistics for Rifle Shooters

Ballistic coefficient variations. Most manufacturers publish only G1 BCs for their bullets. The G1 coefficient doesn't match very well the shape of modern boattail bullets. To accomodate that, Sierra publishes BCs for their bullets in velocity ranges. However, many ballistic programs are not set up to handle multiple BC values. A better match to boattail bullets is the G7 BC, which will produce a better calculation of bullet velocity at range, which is useful to shooters who are operating near the transonic range of their bullets. Some bullets exhibit unpredicable behaviour in the transonic range. The Sierra 168 grain Matchking is one such bullet.

One source of G7 BCs produced by extensive testing is the book Applied Ballistics for Long Range Shooting by Bryan Litz. I highly recommend this book for anyone serious about shooting at extended distances.

In addition, the online program JBM Ballistics has some G7 BCs in its bullet library. It does not tell you what the G7 BC is - just select the a bullet with the label "(Litz)" after the listing in the bullet library. If there is no such label after the bullet you are using, a G7 coefficient is not available. Also select G7 in the main screen.

Range uncertainty. This is a prime cause of differences between the ballistic program output and field shooting data at long range. At 1000 yards with our typical .308 load, a range error of 20 yards will cause an elevation error of about 18 inches, which is 0.5 mil or almost 2 MOA. You may believe your laser rangefinder is accurate - but a one percent error at 1000 yards is 10 yards, and that's assuming that you actually managed to laser the target. The manufacturer of the common Leica 1200 claims an accuracy of +/- 0.5 percent beyond 800 yards.

Elevation variations caused by the headwinds or tailwinds. Headwinds slightly increase the drag on the bullet, and tailwinds reduce it. Not all ballistics programs correctly model this effect.

Aerodynamic jump. This is Bryan Litz's description of this factor: "Aerodynamic jump is what causes groups to slant when shot in varying wind conditions. Basically, when the bullet exits the muzzle into a cross wind, the bullet tries to yaw slightly to align itself with the airflow. When the bullet yaws to the side, gyroscopic action causes it to nose up or down by a small amount depending on the wind direction. This initial yaw has an effect on the trajectory, and is known as aerodynamic jump. The more severe the cross wind, the more pitch the bullet ends up with. Flying to the target at a pitch angle will result in an elevation error that's proportional to crosswind." From: Extending the Maximum Effective Range of Small Arms. That's a good article which describes some of the limitations of existing ballistic programs.

The E�tv�s effect. This is an elevation variation caused by the earth's rotation. It is of most significance on long shots taken directly due east or west. Some ballistics program do not correctly model this effect. Bryan's Litz's book previously referenced has a section on how to calculate that effect if your ballistic program does not. The magnitude of this variation might be in the range of 10 inches maximum difference on a 1000 yard shot between a due east shot and a due west shot, depending on your lattitude.

Incorrect specification of atmospheric parameters. Many shooters do not understand the difference between station pressure and barometric pressure referenced to sea level. See Barometric Pressure and Ballistic Software.

Error in Inclined Shot Calculations. Many ballistic program do not correctly compensate for the difference between an uphill shot, where gravity slightly hinders the bullet, and a downhill shot, where gravity is slightly aiding the bullet. For example, on a 900 yard shot at a 30 degree angle, the difference between an uphill shot and a downhill shot is about one MOA. The magnitude of this difference increases with the angle. Most programs I have seen are doing a calculation appropriate to an uphill shot, so what one might do to compensate for that is to hold a little low on a downhill shot.

An additional possible error on an inclined shot is failure to compensate for the difference in air density between the firing position and the target. For example, on a 900 yard shot at a 30 degree angle, the altitude difference between the firing position and the target is 1350 feet. No ballistic program I am aware of attempts to compensate for that difference.

Parallax error caused by improper adjustment. When looking through the scope at the target, the reticle should not appear to move relative of the target when you make a slight movement of your head. If it does, parallax error is present and should be corrected.

Zero errors. If your hundred yard zero is off by a quarter of an inch, at 1000 yards your point of aim will be off by 2.5 inches.

Shooter variations. We have seen two shooters have different points of impact on the same target at long distance using the same rifle and load. That's because of differences in the way the rifle is held by the shooter.

Differences in atmospheric modeling between the programs.

Now What?

Now that we know some of the causes, what can we do?

We can systematically modify the program output by shooting long-range with our load, and then adjusting the BC input to the program until the program output matches the shooting data. I typically do this at a range of 1000 yards with a .308. Nota bene: it should be done at a range and under conditions where you know the bullet has not entered the transonic region. We might say the bullet has entered the transonic region if its velocity has decreased to within 110 percent of the speed of sound. (The speed of sound is about 1125 feet per second at 68 degrees F. It varies only with temperature.) So, if the speed of sound is 1125 feet per second, we might say the bullet is in the transonic region when the speed has dropped to 1238 fps.

If other sources of error discussed above have not been ruled out, it should be obvious that the correction obtained by this process will apply only to this load fired from one specific rifle with one specific scope - maybe.

Understand from this discussion that no ballistic program can produce an output sufficiently accurate to guarantee a first-round hit at ranges beyond a few hundred yards. A ballistic program is correctly used to get you close to that first-round hit under conditions you don't normally shoot in. An example is training with your rifle and load at sea level, and then trying to make a high-altitude shot.

When someone says that their un-tuned ballistic program was "right on at 1000 yards", I generally conclude that if they are telling the truth, they were lucky enough to have offsetting errors.


link: http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

[Rotnguns]

Good read, but I take issue with the statement about scope clicks.  A minute of angle corresponds to 10.47 inches at 1000 yards.  If a scope click provides 1 inch at 100  yards instead of a MOA, there's only a 0.47 inch difference at 1000 yards:

1/60 degrees X PI rads/180 degrees X 3000 feet X 12 inches/foot = 10.472 inches  per MOA at 1000 yards.

I thought some of you might enjoy this read,

Shooters using ballistic programs often find that their field shooting data does not precisely agree with the output from their program.

In addition, shooters comparing the outputs of different programs often find significant discrepancies between the programs. Why?

Below are listed the cause of some inaccuracies in the elevation predictions of ballistic programs. Windage calculations are beyond the scope of this article.

Uncalibrated scope clicks. For example, Leupold M1 dials I have tested are closer to 1 inch per hundred yards than 1 MOA. With a typical .308 load at 1000 yards, the difference in point of impact is about 20 inches. If you don't know how to check that calibration, you may find this article useful: Optically Checking Rifle Scopes

Normal variations in muzzle velocity. A good load may have a muzzle velocity standard deviation of 15 feet per second. That means that about two-thirds of the shots will fall in the range from 15 fps per second above the average velocity to 15 feet per second below the average. At 1000 yards with a typical .308 load, that range of variations will cause shots distributed over a 10 inch range. Nota bene: About one third of the shots will have variations from the average which are even greater. There are many causes of this velocity variation, and they are beyond the scope of this article.

Temperature variations in muzzle velocity. A pretty good powder will exhibit a variation in muzzle velocity of 1 foot per second per Fahrenheit degree. If you chronographed your load at 85 degrees and are shooting at 55 degrees, your muzzle velocity may be 30 feet per second slower than you think it is. I have measured variations as large as 5 feet per second per Fahrenheit degree. The only way to know how big that variation is, is to test it. In addition, my experience has been that the standard deviation of the muzzle velocity increases at low temperatures.

Muzzle velocity measurement errors. Do you believe your chronograph is accurate? The screen spacing of most chronographs is too short for accurate measurement. Some chronographs like the Oehler 35 series allow extending the spacing. The clock frequency of many chronographs is too slow to get accurate and consistent measurements, i.e., more than one shot with the identical muzzle velocity will be displayed with different measurements. Other common problems with chronograph data is failing to compensate for the distance between the muzzle and the chronograph screens, and failing to fire enough rounds to have a good average velocity. People with little understanding of statistics may have a very vague idea of what the chronograph output means. See Statistics for Rifle Shooters

Ballistic coefficient variations. Most manufacturers publish only G1 BCs for their bullets. The G1 coefficient doesn't match very well the shape of modern boattail bullets. To accomodate that, Sierra publishes BCs for their bullets in velocity ranges. However, many ballistic programs are not set up to handle multiple BC values. A better match to boattail bullets is the G7 BC, which will produce a better calculation of bullet velocity at range, which is useful to shooters who are operating near the transonic range of their bullets. Some bullets exhibit unpredicable behaviour in the transonic range. The Sierra 168 grain Matchking is one such bullet.

One source of G7 BCs produced by extensive testing is the book Applied Ballistics for Long Range Shooting by Bryan Litz. I highly recommend this book for anyone serious about shooting at extended distances.

In addition, the online program JBM Ballistics has some G7 BCs in its bullet library. It does not tell you what the G7 BC is - just select the a bullet with the label "(Litz)" after the listing in the bullet library. If there is no such label after the bullet you are using, a G7 coefficient is not available. Also select G7 in the main screen.

Range uncertainty. This is a prime cause of differences between the ballistic program output and field shooting data at long range. At 1000 yards with our typical .308 load, a range error of 20 yards will cause an elevation error of about 18 inches, which is 0.5 mil or almost 2 MOA. You may believe your laser rangefinder is accurate - but a one percent error at 1000 yards is 10 yards, and that's assuming that you actually managed to laser the target. The manufacturer of the common Leica 1200 claims an accuracy of +/- 0.5 percent beyond 800 yards.

Elevation variations caused by the headwinds or tailwinds. Headwinds slightly increase the drag on the bullet, and tailwinds reduce it. Not all ballistics programs correctly model this effect.

Aerodynamic jump. This is Bryan Litz's description of this factor: "Aerodynamic jump is what causes groups to slant when shot in varying wind conditions. Basically, when the bullet exits the muzzle into a cross wind, the bullet tries to yaw slightly to align itself with the airflow. When the bullet yaws to the side, gyroscopic action causes it to nose up or down by a small amount depending on the wind direction. This initial yaw has an effect on the trajectory, and is known as aerodynamic jump. The more severe the cross wind, the more pitch the bullet ends up with. Flying to the target at a pitch angle will result in an elevation error that's proportional to crosswind." From: Extending the Maximum Effective Range of Small Arms. That's a good article which describes some of the limitations of existing ballistic programs.

The E�tv�s effect. This is an elevation variation caused by the earth's rotation. It is of most significance on long shots taken directly due east or west. Some ballistics program do not correctly model this effect. Bryan's Litz's book previously referenced has a section on how to calculate that effect if your ballistic program does not. The magnitude of this variation might be in the range of 10 inches maximum difference on a 1000 yard shot between a due east shot and a due west shot, depending on your lattitude.

Incorrect specification of atmospheric parameters. Many shooters do not understand the difference between station pressure and barometric pressure referenced to sea level. See Barometric Pressure and Ballistic Software.

Error in Inclined Shot Calculations. Many ballistic program do not correctly compensate for the difference between an uphill shot, where gravity slightly hinders the bullet, and a downhill shot, where gravity is slightly aiding the bullet. For example, on a 900 yard shot at a 30 degree angle, the difference between an uphill shot and a downhill shot is about one MOA. The magnitude of this difference increases with the angle. Most programs I have seen are doing a calculation appropriate to an uphill shot, so what one might do to compensate for that is to hold a little low on a downhill shot.

An additional possible error on an inclined shot is failure to compensate for the difference in air density between the firing position and the target. For example, on a 900 yard shot at a 30 degree angle, the altitude difference between the firing position and the target is 1350 feet. No ballistic program I am aware of attempts to compensate for that difference.

Parallax error caused by improper adjustment. When looking through the scope at the target, the reticle should not appear to move relative of the target when you make a slight movement of your head. If it does, parallax error is present and should be corrected.

Zero errors. If your hundred yard zero is off by a quarter of an inch, at 1000 yards your point of aim will be off by 2.5 inches.

Shooter variations. We have seen two shooters have different points of impact on the same target at long distance using the same rifle and load. That's because of differences in the way the rifle is held by the shooter.

Differences in atmospheric modeling between the programs.

Now What?

Now that we know some of the causes, what can we do?

We can systematically modify the program output by shooting long-range with our load, and then adjusting the BC input to the program until the program output matches the shooting data. I typically do this at a range of 1000 yards with a .308. Nota bene: it should be done at a range and under conditions where you know the bullet has not entered the transonic region. We might say the bullet has entered the transonic region if its velocity has decreased to within 110 percent of the speed of sound. (The speed of sound is about 1125 feet per second at 68 degrees F. It varies only with temperature.) So, if the speed of sound is 1125 feet per second, we might say the bullet is in the transonic region when the speed has dropped to 1238 fps.

If other sources of error discussed above have not been ruled out, it should be obvious that the correction obtained by this process will apply only to this load fired from one specific rifle with one specific scope - maybe.

Understand from this discussion that no ballistic program can produce an output sufficiently accurate to guarantee a first-round hit at ranges beyond a few hundred yards. A ballistic program is correctly used to get you close to that first-round hit under conditions you don't normally shoot in. An example is training with your rifle and load at sea level, and then trying to make a high-altitude shot.

When someone says that their un-tuned ballistic program was "right on at 1000 yards", I generally conclude that if they are telling the truth, they were lucky enough to have offsetting errors.


link: http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

[J Mack]

Good read, but I take issue with the statement about scope clicks.  A minute of angle corresponds to 10.47 inches at 1000 yards.  If a scope click provides 1 inch at 100  yards instead of a MOA, there's only a 0.47 inch difference at 1000 yards:

1/60 degrees X PI rads/180 degrees X 3000 feet X 12 inches/foot = 10.472 inches  per MOA at 1000 yards.

I think his point is if a .308 takes 40 MOA to get to 1000 yards from a 100 yard zero and you have a .047197580733” error per MOA then you would need to compensate for this in your ballistic software.

[J Mack]

More reading on MOA inch error,

  Target, long-range and military shooters, as well as many hunters, use the terms inches or MOA (Minutes of Angle) to describe their group size or the amount of elevation or windage changes made on their scope's dials. For example, "My 5-round group at 100 yards was 1.2" or 1.2 MOA." Or if I shot a 10" group at 1,000 yards, I might say I shot 1 MOA at a 1,000. Inches and MOA are not the same amount, but for close-range shooting most believe they are approximate enough. At longer range, however, there is almost a 5-percent error between inches and MOA. Why?

Minutes of Angle, as the term suggests, has to do with angles. We all know a circle has 360 degrees and each degree can be further broken into 60 minutes. Each minute can be broken into 60 seconds. Using a bit of math allows us to calculate how much an MOA subtends at 100 yards. Doing so, we come up with 1.04719". The digits go a ways beyond that, but if we only go three places, we end up with 1.047", which is nearly 1.05" and the reason I said earlier thinking MOA is the same as inches will result in a 5-percent error.

Error Magnified

This small error results in a bit of confusion since most elevation and windage dials are marked in 1/4" point of impact change at 100 yards per click. Most target dials are subdivided by a larger tic mark every four, allowing the shooter to quickly dial a 1" point of impact change at 100 yards.

This translates to 2" at 200, 3" at 300, and so on. To understand that, let us assume we draw 1" grids on large pieces of paper and hang them at 100, 200 and 300 yards all the way to 1,000 yards.

Place the rifle in a solid rest so it can't move. Put the horizontal crosshair on a line. Then dial a 1" elevation change. You will see at 100 yards the crosshair moved 1", but on the 200-yard grid it moved 2" and on the 1,000 yard grid the horizontal crosshair moved 10". However, if the clicks were in MOA and we dialed one MOA, it would move the crosshair 1.047" at 100 yards and 10x1.047 or 10.47" at 1,000 yards. This translates to just a .047" difference at 100 yards, but a .47" difference at 1,000 yards. That is, the farther the target is away, the more significant the difference between inches and MOA becomes. Granted this difference appears small, but in a moment you will see how it results in a very significant difference in use.

Unfortunately, many shooters use MOA to call out the amount to come up on the elevation dial or to move into the wind when the shooter has 1/4" clicks on the turrets. For example, having sighted in at 100 yards, my comeups for the .308 are 38" at 1,000 yards. If I dialed in MOA, it would be 38 x 1.047 = 39.786". That is, 1.786" x 10 = 17.86" higher. For most hunters shooting in the 300-yard range or less, this error is not often important, but for precision or long-range shooting it is significant enough to result in a miss. The way to get around this is to shoot these ranges and note down the amount the dial must be turned to make these shots. Earlier I said the difference between MOA and inches is 5 percent. How did I make a 17.86" error at 1,000 yards?

The error is in the MOA vs. inches, not in the result. Think about it. One inch at 100 yards is 1". One MOA at 100 yards is 1.047". At 200 yards, if I come up 2" on the dial to go to 200 yards, I have come up 2" at 100 yards. However, If I come up 2 MOA, I have comeup 2.094" at 100 yards. If I come up 11.25" to go to 500 yards, I have come up 11.25". But if I come up 11.25 MOA, I have come up 11.779", etc. So ... as I progress out, I am picking up the 5 percent error, and the result is growing. Why? Let's back up. The error at 100 yards between 11.25" and 11.779" is .529". Thus my error at 200 is twice that or 1.06". At 500 yards it is five times that or 2.645". At 1,000 yards it is 10 times that or 10.6".

Look at it as windage. If I dial in 1" left windage at 100 yards, it will be 10" off at 1,000 yards. The bullet keeps making that 1" angle off clear to 1,000 yards. By the time it gets to 1,000 yards, it is off 10". So suppose I made the mistake of dialing in MOA--1.047". I would be .047" off, which would translate to .47" off at 1,000. But suppose I had to dial 38" of wind, but instead made it MOA, which is 39.786". The error now is 1.786". So let us suppose we just dialed in 1.789". At 1,000 yards the wind error would be 17.89" because the error keeps going by that angle. Most people think the difference between inches and MOA is so small it does not matter, but it becomes significant the farther out we go.

Read The Reticle Right

If a scope has hash marks on the reticle, it is important to know how many inches or MOA they subtend at 100 yards if used for long-range shooting instead of dialing the elevation turret. You might also use them for ranging. For example, you might decide an elk has a 32" chest. Place the main horizontal reticle on the elk's back and, if each hash mark subtends 2" at 100 yards, noting the second hash mark is on the bottom of his chest, one would divide 32 / 4 = 8 x 100 = 800 yards. But if the hash marks were in MOA, the result would be 32 / 4.188 = 7.64 x 100 = 764 yards, a difference of 36 yards (because 4 x 1.047 = 4.188). This would very likely result in a miss.

 So why do spotters call out changes in MOA? If one is using a Nightforce scope with 2 MOA hash marks it would be exact when using the hash mark, but it would not be exact if using the 1/4"-click-at-100-yards elevation dial. The reason is because it is easier to call out MOA. MOA relates a change relative to the range.

For example, suppose my spotter called out 1" when I'm firing at 500 yards. I might think he meant come up 1" on the target or I might think he meant come up 1" on the dial. The first would be a 1" change at 500, the second would mean a comeup 5" on the target. Rather confusing. Or he could call out the exact change at 500 yards as 5". However, if he calls out 1 MOA, I know immediately what he means, because it relates immediately to any range. He means come up 5" on the target.

Luckily, Nightforce had the foresight to put both hash marks and dial clicks in MOA. But caution is appropriate here. Very few scope dials are exact and neither are hash marks. Make sure what yours are by using a grid or by some other means.

Link; http://findarticles.com/p/articles/mi_m0BQY/is_9_53/ai_n27320038/

[Rotnguns]

I may not be understanding his parlance. Seems like he's acknowledged the error at 1000 yards to get 39.786 inches.  Then, he multiplies by 1.786 (diff at 1000 yards due to MOA v 1/4 inch clicks) by 10 again?

More reading on MOA inch error,

Unfortunately, many shooters use MOA to call out the amount to come up on the elevation dial or to move into the wind when the shooter has 1/4" clicks on the turrets. For example, having sighted in at 100 yards, my comeups for the .308 are 38" at 1,000 yards. If I dialed in MOA, it would be 38 x 1.047 = 39.786". That is, 1.786" x 10 = 17.86" higher. For most hunters shooting in the 300-yard range or less, this error is not often important, but for precision or long-range shooting it is significant enough to result in a miss. The way to get around this is to shoot these ranges and note down the amount the dial must be turned to make these shots. Earlier I said the difference between MOA and inches is 5 percent. How did I make a 17.86" error at 1,000 yards?

Link; http://findarticles.com/p/articles/mi_m0BQY/is_9_53/ai_n27320038/

[J Mack]

I may not be understanding his parlance. Seems like he's acknowledged the error at 1000 yards to get 39.786 inches.  Then, he multiplies by 1.786 (diff at 1000 yards due to MOA v 1/4 inch clicks) by 10 again?

I need to work on my copy paste foo!
Reread above, you still my not agree with his logic but at least you can see how it plays out.

[ballardw]

A brief note on standard deviation and velocity range in the first part of this thread: The two-thirds (about 68%) within plus or minus one standard deviation applies if the velocities measured are approximately normally distributed. If for some reason your spreads aren't symmetric around your average velocity it may mean that your velocities are not normally distributed and could get much worse effects than the example lists.

If the method you are using to get standard deviations will also generate skewness and the value is close to zero you'll be closer to a comfort level about the validity of the expected range of effects on target.

We probably do not want to go into statistical tests for normality here though.

[ida83704]

More from Lindy on Barometric Pressure

link has pics and illustrations
http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Baro/


Barometric Pressure and Ballistic Software
by Linden B. (Lindy) Sisk

Last Revision December 3, 2009 

--------------------------------------------------------------------------------

The earth’s atmosphere is essentially a column of air.

As you might expect, because of gravity there is more pressure at the bottom of that column than at the top – since the top is essentially a vacuum, i.e., outer space. At altitudes where humans can live, barometric pressure decreases approximately at the rate of one inch of mercury per 1000 feet of elevation gain.

Air pressure at the bottom of the column, i.e., the surface of the earth, varies as well, with weather. Hurricanes and storms are usually low-pressure systems, while clear weather is usually accompanied by higher pressure. For the purposes of this article, we will refer to that pressure in inches of mercury. If the reader is more accustomed to seeing barometric pressure (BP) data expressed in millibars, note that inches of mercury can be converted to millibars by dividing the reading in inches of mercury by 29.53. If local weather data is reported in millibars, it can be converted to inches of mercury by dividing the number of millibars by 1000, then multiplying by 29.53.

In addition, some programs use either inches of mercury or millibars.

The lowest BP recorded in the western hemisphere was 26.05 inches of mercury on October 19, 2005, during Hurricane Wilma. The highest pressure recorded in the world was 32.01inches at Agata, USSR, on 31 December, 1968.

What is referred to as “Army Standard Metro Conditions”, used in ballistic calculations, is a BP of 29.53 inches of mercury at a temperature of 59 degrees F. with a relative humidty of 78 percent. That will come in handy later.



--------------------------------------------------------------------------------


Ballistic Software
In order to calculate the elevation data necessary for accurate shooting, ballistic software must model the density of the air. Air density is computed using three inputs: atmospheric pressure, temperature, and the moisture content of the air. That means that you the user must specify those quantities to the program. Ballistic programs usually are programmed to input moisture data as a relative humidity figure.

Atmospheric pressure comes in two flavors, which we will refer to as station pressure and barometric pressure.

Station pressure for our purposes refers to the absolute atmospheric pressure at the measurement point.

Barometric pressure refers to the atmospheric pressure reported as if the reporting station is at sea level, irrespective of its actual altitude, referred to hereafter as SLBP. This is the kind of pressure reported by the National Weather Service and other weather reporting agencies and services. This is done so that people will have a common reference for the pressure.

If you live at or near sea level, as I do much of the year, barometric pressure and station pressure are the same. It’s only at altitudes higher than sea level that the difference becomes significant.

Leadville, Colorado, is situated at just above 10,000 feet. That means when the barometric pressure is 29.52 inches of mercury, the station pressure at Leadville is around 19.5 inches of mercury. The pressure is reported as if Leadville were at sea level, so that reports of atmospheric pressure of less than 20 inches of mercury won’t send people screaming into the street thinking that they are about to die. But the actual station pressure is, in fact, less than 20 inches of mercury, which causes some stress in people running the annual Western States 100 footrace in the nearby mountains – a 100 mile ultramarathon at high altitudes.


Getting the Pressure
Now that you understand the difference between station pressure and barometric pressure, we can deal with finding the pressure.

Most software packages can deal with either station pressure or barometric pressure. If you specify an altitude and the barometric pressure, the software can use that information to calculate the approximate station pressure, which is what is needed for a ballistic calculation. It’s usually, however, a bit more accurate to enter the station pressure, if you know that.

Station pressure can be obtained from a variety of instruments. A number of outdoor watches have pressure sensors, as do instruments like the Kestrel family of meteorological instruments. Make sure that what you are getting is the station pressure. With a Kestrel and similar instruments, there is a reference altitude setting in the barometric pressure window. If you leave that reference altitude set to zero, what it displays is the station pressure.

If you set the reference altitude in the Kestrel to the local altitude, it will use those numbers to calculate the barometric pressure as reported by the weather bureau, i.e., referenced to sea level.

Don’t do that – unless you want to compare the numbers to check the calibration of your instrument …or the weather bureau’s. Remember the old saying that a man with one watch knows what time it is, while a man with two watches is never quite sure – unless he has an atomic clock, but, I digress. It’s the same with two pressure figures. They will be different – but probably not different enough to matter.

If you are using some other kind of pressure sensor, examine the figure bearing in mind the approximate pressure lapse rate of 1 inch of mercury per 1000 feet. If you’re at 10,000 feet, and your watch displays a pressure of 29.9 inches, it’s displaying a barometric pressure, rather than a station pressure.

Barometric pressure might be obtained from a local radio or television station, from the Automatic Terminal Information System (ATIS) broadcast at a nearby airport, or from the Internet.

Entering the Pressure into Ballistic Programs
A popular online ballistic program is JBM Ballistics. Down near the bottom of the screen are the atmospheric parameters, which look like this:

 

If you check "Standard Atmosphere at Altitude", you get the ICAO temperature and pressure at whatever altitude you specify, with the humidity set to zero.

That means that whatever temperature and pressure you might have entered in the input screen are completely ignored, and humidity is set to zero. For your reference, an ICAO Standard Atmospheric chart is included at the bottom of this article.

If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects that pressure for altitude.

That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate boxes.

If "Pressure is corrected" is not checked, the density is calculated ignoring the contents of the altitude box. In other words, the programs assumes that you have entered the station pressure. It still uses the entered temperature and relative humidity in the air density calculation.

Nota Bene: Do not check both the "Std. Atmosphere at Altitude" and the "Pressure is Corrected" boxes.

Exbal and Nightforce
Below is the Field Conditions entry screen from the PDA version of Exbal current at this time. There are various version of Exbal and Nightforce, but, with the exception of the inclusion of the density altitude option, all should work the same.



--------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------

If you know the station pressure, uncheck the "Calculate Standard Pressure" box, and enter the station pressure in the Pressure box, as well as the temperature and the humidty. With that box unchecked, whatever is in the altitude box is ignored in the calculation.

If what you know is the SLBP, enter that into the Pressure box, and enter your actual altitude, as well as the temperature and relative humidity. Check the box which says, "Calculate Standard Pressure."

If what you wish to have is a calculation for a density altitude, enter the density altitude in the Altitude box, and check the Use Density Altitude box. As shown below, by the graying out of the temperature, pressure, and relative humidity boxes, those come from the ICAO Standard Altitude, and are not available as inputs. Note that they agree with the values in the ICAO Standard Atmosphere Chart shown below.



--------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------

Field Firing Solutions
Below is the Presets screen from Field Firing Solutions. It works similarly to Exbal. If you have the station pressure, enter it in the Pressure box, and check the Stat box as shown:



--------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------
If what you have is the SLBP, enter it in the Pressure box and check Baro, as below. In either case, the program will use the locally entered temperature and relative humidity.


--------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------


What If I Have Some Other Program?
Then you'll have to figure out how to enter the atmospheric parameters.

A good way to ensure that you have a good understanding is to experiment. Enter the environmental parameters you usually shoot in, and perform a calculation for a range you have actual shooting data for, preferably for a target which is more than 500 yards distant.

Then change the station pressure figure to, say, 5 inches of mercury less than what you usually have. When the program performs the calculation, you should note a marked decrease in the calculated elevation.

Also experiment with getting your program to perform the conversion from barometric pressure to station pressure. If, for example, you put in a condition of barometric pressure of 29.85 at an altitude of 6000 feet, and compare that with the results of a calculation made using a station pressure of 23.85 at 6000 feet, you should see approximately the same results from the elevation calculation. If not, you’ve goofed.

Change the temperature and relative humidty separately, and ensure that the program is changing the output appropriately, remember than a higher temperature should result in a lower required elevation setting for a given shot, and that a higher relative humidity should also result in a lower required elevation setting.

Experiment, bearing in mind the difference between station pressure and barometric pressure, and you’ll figure it out.

Remember this: the most accurate way to get any ballistic program to work is to have an instrument like a Kestrel or many watches which measures local atmospheric pressure, which we call the station pressure. Enter the station pressure, the temperature, and the relative humidity, and make sure that you're not using a program option to calculate the "standard atmosphere".

Good Shooting!



--------------------------------------------------------------------------------

 


--------------------------------------------------------------------------------

This file is: http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Baro/index.html


© 2007, 2008, 2009 by Linden B. (Lindy) Sisk
Permission is granted to print or photocopy the entire article intact, including this notice. All other rights reserved.