29 January 1998 stable



BULLET STABILITY CALCULATOR

CALCULATOR

This spreadsheet calculates MAXIMUM bullet length, MINIMUM velocity, and MINIMUM rate of twist, given the values of the variables that you dictate.

There are four blocks for calculations. Bullet diameter and weight are required in all cases. Any combination of the other three variables can be used to solve for the missing fourth variable. To make this spread sheet more user-friendly and to cross-check calculations the data only needs to be entered into the one calculation. It is then automatically put into the other calculation blocks so all four answers are provided at the same time.

A bullet is spin-stabilized by rotating it about its long axis. The amount of spin required is determined by the specific gravity, shape and construction of the bullet, bullet velocity, and the density of the air it is going to travel through. The force, or overturning moment, acting on the bullet nose is greatest when the bullet first exits the barrel. If a certain bullet-barrel combination will stabilize a bullet at the muzzle, the bullet will remain stable for the rest of its flight. As a bullet flies on toward the target it is losing velocity rapidly as any trajectory table will show. The rotational speed of the bullet or its RPM's decreases at a much slower rate. The overall result is a lessening force acting on the bullet nose, and that force is being overcome by a proportionately greater spin rate compared to forward velocity. The down-range bullet therefore is more stable than it was at the muzzle.

Changes in air density also have an effect on the twist rate required to stabilize a bullet. That is, when air density decreases, there is a decrease in the amount of spin required to keep a bullet flying point on. Conversely, when the density of the air increases, more spin is required.

The measure of projectile stability is known as the stability factor, or SG. A value of 1.0 describes a bullet that is just barely stable. If the value drops below 1.0 a bullet is unstable; it yaws wildly. Accuracy is nonexistent. Obviously then, the stability factor must be at some measurable level above the absolute minimum of 1.0. It has been determined that a stability factor of 1.3 is the minimum level for reliable accuracy and performance. Some authorities suggest that the stability factor should be no less that 1.5 for any application.

As the level of stability goes down, any minor change in the rifle-bullet combination could cause a wild shot. For example, a damaged muzzle or crown could cause a bullet to leave the barrel with enough yaw that it cannot recover in flight. At worst, the bullet will completely lose stability, or it may not hit the target in the intended spot. An excessively fouled bore may also have enough effect to cause these problems, as could a badly imbalanced bullet.

Explanation of inputs:

Stability Factor (SF) : should be >= 1.00 for the bullet to be stable - the higher the SF, the better.

Velocity (V) (feet per second): the velocity of the bullet. The higher the velocity, the more stable the bullet will be.

Length of Bullet (BL) (inches): overall length of the bullet. The longer the bullet, the more difficult it is to stabilize.

Weight of Bullet (WT) (grains): bullet weight in grains.

Rate of Twist (TWIST) (inches per turn): the twist rate of the barrel. The lower the number, the higher the rate of twist (the quicker the spin). Quicker twists (1:7) stabilize bullets better than slower twists (1:14).

Bullet Diameter (DIA) (inches) : the bullet diameter in inches, or caliber.

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