5/13/2014 Ballistic Program Inaccuracies

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/ 1/4

Sources of Ballistic ProgramInaccuracies

by Linden B. (Lindy) Sisk

Last Revision July 3, 2010*

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

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

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

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

Normal variations in muzzle velocity. A good load may have a muzzle velocity standard deviation of 15feet per second. That means that about two-thirds of the shots will fall in the range from 15 fps per secondabove 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 theshots will have variations from the average which are even greater. There are many causes of this velocityvariation, and they are beyond the scope of this article.

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

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

Ballistic coefficient variations. Most manufacturers publish only G1 BCs for their bullets. The G1coefficient doesn't match very well the shape of modern boattail bullets. To accomodate that, Sierrapublishes BCs for their bullets in velocity ranges. However, many ballistic programs are not set up to handlemultiple BC values. A better match to boattail bullets is the G7 BC, which will produce a better calculation

5/13/2014 Ballistic Program Inaccuracies

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/ 2/4

of bullet velocity at range, which is useful to shooters who are operating near the transonic range of theirbullets. Some bullets exhibit unpredicable behaviour in the transonic range. The Sierra 168 grain Matchkingis one such bullet.

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

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

Range uncertainty. This is a prime cause of differences between the ballistic program output and fieldshooting data at long range. At 1000 yards with our typical .308 load, a range error of 20 yards will causean elevation error of about 18 inches, which is 0.5 mil or almost 2 MOA. You may believe your laserrangefinder is accurate - but a one percent error at 1000 yards is 10 yards, and that's assuming that youactually 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 thebullet, 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 causesgroups to slant when shot in varying wind conditions. Basically, when the bullet exits the muzzle into across 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. Thisinitial yaw has an effect on the trajectory, and is known as aerodynamic jump. The more severe the crosswind, the more pitch the bullet ends up with. Flying to the target at a pitch angle will result in an elevationerror 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 Eötvös effect. This is an elevation variation caused by the earth's rotation. It is of most significanceon 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 ballisticprogram does not. The magnitude of this variation might be in the range of 10 inches maximum differenceon 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 differencebetween station pressure and barometric pressure referenced to sea level. See Barometric Pressure andBallistic Software.

Error in Inclined Shot Calculations. Many ballistic program do not correctly compensate for thedifference between an uphill shot, where gravity slightly hinders the bullet, and a downhill shot, wheregravity is slightly aiding the bullet. For example, on a 900 yard shot at a 30 degree angle, the differencebetween an uphill shot and a downhill shot is about one MOA. The magnitude of this difference increaseswith the angle. Most programs I have seen are doing a calculation appropriate to an uphill shot, so what onemight 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 densitybetween the firing position and the target. For example, on a 900 yard shot at a 30 degree angle, the altitudedifference between the firing position and the target is 1350 feet. No ballistic program I am aware of

5/13/2014 Ballistic Program Inaccuracies

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/ 3/4

attempts to compensate for that difference.

Parallax error caused by improper adjustment. When looking through the scope at the target, the reticleshould 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 willbe off by 2.5 inches.

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

Differences in atmospheric modeling between the programs.

Now What?

Now that we know some of the causes, and have eliminated or compensated for as many of the variables aswe can, what can we do?

We can systematically modify the program output by shooting long-range with our load, and thenadjusting the muzzle velocity or BC input to the program until the program output matches the shootingdata. I typically do this at a range of 1000 yards with a .308. Nota bene: it should be done at a range andunder conditions where you know the bullet has not entered the transonic region. We might say the bullethas 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, ifthe speed of sound is 1125 feet per second, we might say the bullet is in the transonic region when thespeed has dropped to 1238 fps.

If other sources of error discussed above have not been ruled out, it should be obvious that the correctionobtained 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 toguarantee a first-round hit at ranges beyond a few hundred yards. A ballistic program is correctly used toget you close to that first-round hit under conditions you don't normally shoot in. An example is trainingwith 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 concludethat if they are telling the truth, they were lucky enough to have offsetting errors.

* Revision History:

July 3rd, 2010. All that was changed was a link to the Engleman article on chronograph statistics.

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

© 2009, 2010 by Linden B. (Lindy) Sisk

Permission is granted to print or photocopy the entire article intact, including this notice. All other rights

5/13/2014 Ballistic Program Inaccuracies

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/ 4/4

reserved.