Calculators

NOTE: JBM will not guarantee these calculations for ANY purpose. Remember to use common sense at all times and to check loading with current manufacturer's data.

If you think the functionality has changed, check the list of changes because it probably has! Questions about calculations may be answered on the FAQ.

General Trajectory Calculators

• Trajectory
  Calculates the trajectory from bullet BC and firearm info. Inputs include muzzle velocity, sight heights, chronograph distance, ballistic coefficient, drag function, line of sight and cant angles and atmospheric conditions. Output options include variable ranges and choice of units for windage and drop (inches, MOA, and mils). Calculations are performed assuming the bullet is a point mass. Elevation and azimuth are iteratively corrected to ensure a correct zero.
• Trajectory -- Drift
  This is almost exactly like the Trajectory calculator above, but it includes inputs for barrel twist, twist direction and bullet length. These values are used to calculate spin drift using an approximation developed by Bryan Litz and published in his book, Applied Ballistics for Long Range Shooting. The stability factor is calculated using the Miller stability formula. When using the bullet library, you must still input the bullet length because it is not in the library.
• Trajectory -- Simplified
  Many people don't use all the terms in the trajectory calculation page listed above, so this program uses the default values for inputs not set by the user. The calculation is done with the same program as the trajectory page above.
• Modified Point Mass Trajectory
  Calculates a trajectory from bullet dimensions. This type of trajectory provides a good estimate for things like spin drift and stability. I have also added coriolis effects. It has a considerable number of inputs, so beware. This online calculation replaces my previously available MPM program.

Range Cards

Range cards calculate bullet drop for atmospheric conditions that are different than the zero conditions. They do this by calculating a trajectory at the zero conditions and then applying the elevation angle at zero to different atmospheric conditions. For more information see this topic.

All of these calculators find the muzzle velocity at the shooting temperature by using linear interpolation given two muzzle velocities and two temperatures.

All of these calculators include spin drift as an option.

• Trajectory Cards
  A range card for multiple temperatures and altitudes. The output is a matrix of values for bullet drop and windage as a function of temperature and altitude density.
• Trajectory μCards
  A small range card. Most of the inputs are the same as the large trajectory card, but it also includes target direction and speed. Cant is not included, because it makes the target lead pretty complicated (if not useless). Output includes only the notes that you enter and the table. The table is similar to the old card -- drop, windage and lead in two units.
• Trajectory μCards Wind Only
  A small range card with three columns of wind values only -- no drop or target lead. This card does not include drift in the wind values.
• Trajectory μCards - Constant Drop
  A small range card but with regular drop intervals. It does this by finding ranges for which the drop is an even multiple of the desired values (e.g. every minute of angle). This causes the ranges to be irregular. The windage is displayed for the range and input wind speed.

Maximum Range

• Maximum Distance
  Calculates the maximum range a bullet can travel given the muzzle velocity, ballistic coefficient, weight and atmospheric conditions.

Ballistic Coefficient Calculation and Conversion

• Ballistic Coefficients (Time)
  Calculates the ballistic coefficient for a near velocity, time of flight, atmospheric conditions and drag function.
• Ballistic Coefficients (Velocity)
  Calculates the ballistic coefficient for near and far velocities, atmospheric conditions and drag function.
• Bullet Drag and Twist
  Calculates the bullet CD, and CD components, BC required twist and stability for input twist. Inputs required are the bullet measurements including nose length, total length, boattail length, meplat diameter, base diameter, caliber, weight, atmospheric conditions and drag function. This algorithm is based on the McDrag work done by Robert McCoy. See bibliography
• Drag Function Conversion
  Converts a single ballistic coefficient for one drag function to a ballistic coefficient for another drag function. Also calculates the sectional density and form factors.
• Drag Function Array Conversion
  Converts an array of ballistic coefficients to available drag functions and recommends a single BC and drag function to use. This is most commonly used to convert multiple BCs to the best single BC.

Power Factor and Recoil

• Power Factor
  Calculates the power factor and checks divisions for IDPA, IPSC, TSA, USPSA and SASS. Output shows whether or not the division is made (green/red) and by how much in percent.
• Recoil
  Calculates the free recoil energy and velocity using firearm weight, charge weight, bullet weight and firearm velocity.

Stability

• Stability
  Calculates the Miller stability factor. This formula was derived by Don Miller and published in Precision Shooting. This formula is much better than the antiquated Greenhill's formula. Stability value should be in the range of 1.3 to 2.0 to ensure bullet stability. Don Miller and Dave Brennan (editor of Precision Shooting) have also been kind enough to let me host his stability paper on my exterior ballistics bibliograpy page. The paper is titled A New Rule for Estimating Rifling Twist An Aid to Choosing Bullets and Rifles.