JBM - Calculations - Trajectory Card Terms

Input Data

Altitude

Used in estimating the atmospheric conditions. This value is not used unless the Standard Conditions at Altitude checkbox is checked.

Azimuth

The horizontal (side to side) angle the barrel makes with the line of sight. This value is not generally known and can be ignored if the "Azimuth Correction for zero range" option is checked.

Azimuth Correction

If checked, the azimuth is changed to make the bullet cross the line of sight at the zero range -- zero windage at the zero range.

Ballistic Coefficient

The ballistic coefficient for the bullet being used. It is almost always obtained from the manufacturer, but can also be measured. The ballistic coefficient is represents the ratio of the drag of the standard (e.g. G1 standard bullet) bullet to that of the bullet you are shooting. A ballistic coefficient of 0.100 means that your bullet has 10 times the drag of the standard bullet.

Bullet Library

A drop down list showing all the buttons in the library. If a bullet is selected, the trajectory is run with the selected bullet. If "None" is selected, the entered BC, drag function and weight are used.

Bullet Weight

The weight of the bullet in grains (7000 grains per pound).

Corrected Pressure

Check this box if the pressure entered is a corrected station pressure. If it is an absolute pressure, leave this unchecked.

Drag function

A drag function (or G function) provides the forces on a standard bullet for which the drag function was derived. A bullet's ballistic coefficient then relates the drag on any bullet to that of the standard bullet.

Drop and Windage units

Sets the units used to display the drop and windage values in the calculated data. There are two inputs, one for the first column of drop, windage and lead and one for the second column of each. The first column is a linear measurement. The second is an angular measurement.

Elevation

The vertical angle the barrel makes with the line of sight. This value is not generally known and can be ignored if the "Elevation Correction for zero range" option is checked.

Elevation Correction

Or Drop correction, causes the elevation to be raised as required to cause the bullet to cross the line of sight at the zero range. Without this box checked, the zero range is ignored.

LOS Angle

The angle between the line of sight and the ground.

Maximum Range

The maximum displayed range.

Minimum Range

The minimum displayed range. Data is calculated from the muzzle the maximum range, but may be displayed from the minimum to the maximum range. (e.g. with a minimum of 990, maximum range of 1010 and a range increment of 10 you would see data 990, 1000, 1010 yards).

Muzzle Velocity

The velocity of the bullet as either (1) measured at some distance from the muzzle or (2) as estimated from reloading data. The on-line ballistics page can correct for the distance to chronograph. (The velocity lost traveling to the chronograph.)

Notes

Test displayed at the top of the card. Typically used for firearm description.

Pressure

The barometric pressure. It may be corrected or uncorrected depending on the state of the "Corrected Pressure" checkbox.

Range Increment

the increment used in displaying the calculated data.

Ranges in Meters

When checked, all ranges entered (minimum, maximum, increment and zero) are in meters.

Relative Humidity

Relative humidity (percent)

Sight Height

The height of the sight (or scope) above or below from the centerline of the barrel. This is required because the bullet starts at a drop of -[Sight Height].

Sight Offset

The offset of the sight (or scope) to the left or right of the centerline of the barrel.

Standard Conditions at Altitude

When checked, the temperature, pressure and humidity for standard atmospheric conditions at the entered altitude (always 0% at standard conditions) are entered in the calculation. The standard used is the ICAO standard atmosphere.

Target Speed

Speed of a moving target.

Temperature

The measured or estimated air temperature.

Wind Speed

The speed of the wind.

Zero Height

The height of the zero point at the zero range.

Zero Range

The range at which you wish the bullet to cross the line of sight.

Output Data

A

Altitude

BC

The ballistic coefficient

CAL

Bullet diameter

Drop

Bullet drop in specified units

H

Relative humidity, percent

Lead

Target lead for input target speed and range.

LOS

Line of sight angle

MV

Entered muzzle velocity (it will be corrected for chronograph distance, CH, if the chronograph distance is greater than zero).

P

Absolute atmospheric pressure

PC

Corrected atmospheric pressure. This is displayed if the "Correct Pressure" option was checked. If not, the pressure is displayed as "P".

Range

The for the specified drop and windage

SH

Sight height

T

Temperature

WGT

Bullet weight

Wind

Bullet windage in specified units

WS

Wind speed

ZH

Zero height