JBM - Calculations - Drag/Twist Terms

Input Data

Barrel Twist

The twist rate (inches per turn) of the barrel. This value is used to calculate stability. It should not be confused with the required twist calculated by Bullet Drag.

Base Diameter

The diameter of the base of the bullet. For boattails, this is smaller than the caliber.

Boattail Length

The length of the boattail from the rear of the bearing surface to the base of the bullet. It is zero for flat base bullets.

Bullet Specific Gravity

The density of the bullet, relative to the density of water (1.0). 10.7 is a commonly used value for jacketed bullets.

Bullet Weight

The weight of the bullet.

Caliber

Diameter of the bullet's bearing surface (that part that contacts the rifling of the barrel), measured in inches.

Drag Function

The drag function to use to calculate the ballistic coefficients. Most commercial bullets use the G1, but for some long range bullets, the G7 or G8 is typically a better choice. Some military bullets fit the G5 and G6 functions better than the G1. The GL is for high drag bullets and won't really fit anything that Bullet Drag can calculate coefficients for. GI is the Ingall's drag function and is thrown in for fun.

Meplat Diameter

The diameter of the flat portion of the tip of the nose of the bullet. For some bullets, the meplat diameter is zero (or very close).

Muzzle Velocity Units

Units used to calculate muzzle velocity on output.

Nose Length

The length of the nose from the front of the bearing surface to the tip.

Nose Type

One of Secant, Tangent or Conical. A secant nose is one in which the center of the circle describing the ogive is located behind (away from the nose) the point of intersection of the ogive and the body of the bullet. A tangent ogive is tangent to body at the point of intersection of the ogive and body of the bullet. A conical nose shaped like a cone. It's radius is effectively infinite.

Ogive Radius

The radius of the nose ogive. For conical bullets, this value is not used since the ogive radius is essentially infinite.

Rotating Band Diameter

The diameter of the rotating band, if any. Most small arms bullets do not have rotating bands (although the rear of the bearing surface may be 1/10000's larger than the caliber, Bullet Drag Calculations do consider this a rotating band) and therefore the rotating band diameter is equal to the caliber.

Total Length

The total length of the bullet from the base to the meplat.

Output Data

Ballistic Coefficient

The ballistic coefficient calculated using the drag function you selected.

CD Boattail

Boattail drag coefficient.

CD Base

Base drag coefficient.

CD Nose

Nose drag coefficient.

CD Rotating Band

Drag coefficient due to the rotating band, if any.

CD Skin

Component of the drag coefficient due to the skin drag.

CD Total

The bullet's drag coefficient. It is the sum of the components of the drag coefficient above.

Form Factor

The form factor for the bullet. This is a ratio of the actual CD of the bullet to that of the CD of the "standard" bullet for the selected drag function.

Mach Number

Mach number for the given velocity and atmospheric conditions.

Muzzle Velocity

Calculated muzzle velocity for the mach number at standard conditions.

Stability

Stability factor for the bullet at the given muzzle velocity. It should be greater than 1.5 to ensure stability. These stability factors are calculated with the twist you entered since the required twist would always give 1.5! The background will be green when the bullet is stable and red when it is not.

Sectional Density

Bullet sectional density

Twist

The required twist to achieve a stability of 1.5. The background will be green when the bullet is stable and red when it is not.