JBM - Calculations - Trajectory (Basic) Terms

Input Data

Altitude

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

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).

Cant Angle

The angle of the tilt of the firearm to the right or left from vertical. Positive angles are to the right.

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 Relative to Target

Check this box when calculating a trajectory with non-zero cant and drop and windage relative to the target is desired. If the cant is zero, this has no effect.

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 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.

"Energy Column"

Use this input to select what is calculated for "energy" column (fourth column from the right). Values include Energy in foot-pounds or Joules, momentum in pound-seconds or Newton-seconds, Taylor's Knockout Value, optimum game weight for hunting or varmint bullets and Krupp's tank formula in inches and millimeters. Krupp's tank formula gives the thickness of a sheet of mild construction steel that can be penetrated by a full metal jacketed, lead core round. It does not calculate the the penetration depth into a solid (semi infinite) block of steel.

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.)

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]. The sight height must be less than the vital zone radius, 5 inches.

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

The speed of the target in miles per hour. This value is used to calculate running target leads. Negative speeds are to the shooters left, positive to the shooter's right.

Temperature

The measured or estimated air temperature.

Wind Speed

The speed of the wind.

Zero Range

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

Output Data

Atmospheric Density

The calculated density of the air.

Azimuth

The angle that the barrel makes with the line of sight in a horizontal direction -- positive to the shooter's right. This is the "lead" angle in minutes of angle for the selected zero range.

Drop

The distance above or below the line of sight. Listed in either inches, minutes of angle or mils (6400 mils per 360 degrees or 3.375 mils per MOA). Drop is positive above the line of sight and negative below the line of sight.

Elevation

The angle that the barrel makes with the line of sight in a vertical direction. This is the "holdover" angle in minutes of angle for the selected zero range.

Energy

Kinetic energy of the bullet. The kinetic energy is a measure of the maximum amount of work (force time distance) an object can do. [It is also a measure of the work done by the firearm/case/primer/powder on the bullet.]

Lead

The required target lead to the left or right of the line of site to hit a moving target. Lead is positive to the shooter's right and negative to the shooter's left.

Maximum Point Blank Range

Maximum range for which the bullet does not go above or below the maximum point blank range radius. The maximum point blank range is calculated using a default value of 5" for the vital zone radius.

Maximum Point Blank Range Zero

Zero range to achieve the maximum point blank range. The maximum point blank range zero is calculated using a default value of 5" for the vital zone radius.

Range

Distance from the shooter along the line of sight.

Range at Max. Height

This is the range at which the bullet reaches its maximum height in the vital zone when doing the maximum point blank range calculation. This is NOT the maximum height of the bullet for the trajectory calculation.

Sectional Density

Bullet sectional density

Speed of Sound

The calculated speed of sound.

Time

The time of flight of bullet in seconds.

Velocity

The speed of the bullet in feet per second.

Windage

The distance to the right or left of the line of sight. Windage is positive to the shooters right and negative to the shooters left.