JBM - Calculations - Trajectory Terms

Input Data

Altitude

Used in estimating the atmospheric conditions. This value is not used unless the "Standard Conditions at Altitude" checkbox or the "Pressure is Corrected" 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 (Use entered BC)" is selected, the entered BC, drag function and weight are used. Lapua bullets with "(CD)" in the description use the Drag Coefficients supplied by Lapua instead of the ballistic coefficient. Bullets with "(Litz)" in the description use Bryan Litz' BCs.

Bullet Weight

The weight of the bullet.

Caliber

Diameter of the bullet.

Cant Angle

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

Distance to Chronograph

The distance from the muzzle of the firearm to the chronograph used for muzzle velocity measurements. If the muzzle velocity is estimated, enter 0.0 for this value -- no correction.

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 Units 1^st Column

A drop value (the number you enter) and its associated units. Output windage for the first column are multiples of this value/units combination. See the drop units topic for more information.

Drop Units 2^nd Column

Same as Drop Units 1^st but for the second column of drop, windage and lead.

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 for Zero Range

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" Formula

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.

Humidity

Relative humidity (percent)

Include Extra Rows

If checked, the output will include extra rows even if they are not an multiple of the range increment. Extra possible rows include the sound barrier crossing, zero range and maximum point blank range.

Line of Sight Angle

The angle between the line of sight and level ground. This value is used when shooting uphill or downhill. Value is a positive angle when shooting uphill and a negative angle when shooting downhill.

Mark Sound Barrier Crossing

If checked, the output velocity background color is red for the first printed velocity that is less than the calculated speed of sound.

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.

Pressure is Corrected

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

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.

Round Output to Whole Numbers

Removes any part of the drop, windage or lead after the decimal point. This is useful when your drops match the scope click settings and you want the output to be in number of clicks.

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.

Std Atmosphere 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 Angle

The angle of the target speed direction with the line of sight. A 90 degree angle is a target moving perpendicular to the line of sight. This angle is given in degrees by selecting an angle from the list.

Target Relative Drops

Calculates the bullet drop relative to the target and not the line of sight. These values are only different when the cant angle is non-zero.

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.

Vital Zone Radius

Radius of the vital zone for which the maximum point blank range is calculated. The vital zone is defined as the area from one radius below the line of sight to one radius above the line of sight.

Windage

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 "Windage Correction for zero range" option is checked.

Windage Correction for Zero Range

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

Wind Angle

Angle of the wind direction. A wind blowing downrange has an angle of zero, a wind blowing to the shooter's right has an angle of 90, a headwind, an angle of 180 and a wind blowing to the shooter's left has an angle of 270 degrees.

Wind Speed

The speed of the wind.

Zero at Max. Point Blank Range

If checked, the zero range is changed to achieve the maximum point blank range.

Zero Height

The height of the zero point at the zero range.

Zero Offset

The lateral displacement 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

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.

Maximum Point Blank Range Zero

Zero range to achieve the maximum point blank range.

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.