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1. Multiple Ballistic Coefficients

Ballistic coefficients are determined by velocity tests that the manufacturers conduct. The rates of loss are compared to the Ingall's G1 tables to establish a ballistic coefficient. The standard projectile used for building the model used in the Ingall's tables is a one pound, one inch diameter, two caliber ogive point. The aerodynamic shape of more efficient target bullets is different than the standard projectile so that the loss data does not fit the entire velocity range. However, by using several ballistic coefficients to cover the entire velocity range a very good fit can be obtained. This is illustrated by the following example of a Sierra 30 caliber 168 grain MatchKing bullet:

Ballistic Coefficient	Lower Velocity Limit	Velocity Range
0.462	2600	> 2600 fps
0.447	2100	2600 to 2100 fps
0.424	1600	2100 to 1600 fps
0.405	0	< 1600 fps

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2. Wind Data

Wind data consists of two components, a magnitude (velocity), and an approach direction. The convention used by Exbal is that the line of sight is zero degrees. A wind approaching from the 3 o'clock position has an angle of 90 degrees.

3. Atmospheric Pressure

Atmospheric pressure is determined by measuring the ambient, absolute pressure with a barometer. A mercury barometer is the standard scientific method for measurement. If the altitude is above sea level, barometric pressure readings are commonly "corrected" for sea level for reporting purposes to establish a common basis. When you hear a weather report for example, the barometric pressure is corrected for sea level.

Another source for atmospheric pressure data is an electronic pressure gauge. These devices report the absolute pressure and do not correct for sea level.

The Exbal program allows you to use either of these conventions. The "pressure at sea level" box is for corrected readings and the "pressure at altitude" is for uncorrected readings. If you specify pressure at sea level, the program will automatically calculate the pressure at altitude, and vice versa.

If you are equipped with an electronic pressure gauge, thermometer, wind velocity meter, and Exbal loaded on a lap top computer, you will have a definite advantage at the range.

4. Separate Sight-in and Range Conditions

A unique feature of Exbal is the ability to establish separate sight-in and range conditions. This is illustrated by the following example:

	Sight-in Conditions	Range Conditions
Location	Corpus Christi, TX	DuBois, WY
Altitude	30 ft	7500 ft
Temperature	80 degrees F	40 degrees F
Muzzle Velocity	3100 fps	3050 fps
Relative Humidity	78%	78%
Pressure corrected to sea level	29.53 inches Hg	29.53 inches Hg
Pressure at altitude	29.53 inches Hg	22.70 inches Hg

This example corresponds to some very practical considerations when preparing for a hunting trip or a match at a different location where there may not be an opportunity to sight-in. First, there is a significant change of altitude and temperature. Notice that with the change in altitude there is a change in atmospheric pressure at that altitude. Finally, there is a decrease in velocity because the powder being used is temperature sensitive (some powders are not). All of these factors affect the trajectory of the bullet. At two hundred yards or less, it probably won't make much difference. Beyond that, the difference in atmospheric conditions becomes increasingly more important as the distance to the target increases. At one thousand yards, these differences would mean 30 inches or 3 minutes of angle for a 300 Winchester Magnum. To a competitor, this is enormous.

Exbal gives you the capability to pinpoint the trajectory calculations to give you a cutting edge advantage.

5. Interface to Excel

Exbal has the ability to load an Excel spreadsheet with the ballistic calculation results. In the default case, a spreadsheet with six sheets is used. After the first sheet is loaded with calculations, you can click on the tab for sheet 2 and run another calculation. When you load the Excel spreadsheet this time, the results will go into sheet 2. This process can be repeated for all six sheets. If you need more, there is an option to set the maximum number of sheets in the workbook.

This interface provides all the power of Excel to analyze data, create graphic displays, etc.

Last Update: Thursday, October 03, 2002 10:30 AM