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Gyroscopic Stability Factor
Q. Would you please explain the term "gyroscopic stability factor" for projectiles? How is it calculated, and what do the numbers represent?
A. Gyroscopic stability factor is a dimensionless number derived from the classical equation:
Sg= (A2*N 2) / (4*B*Km*Ro*D 3*u)
where Sg = gyroscopic stability factor
A = axial moment of inertia
N = rate of spin
B = transverse moment of inertia
Km = moment coefficient
Ro = air density
D = projectile diameter
u = instantaneous velocity of projectile
Theoretically, for a projectile to be stable it must have a stability factor equal to or greater than 1.0. While projectiles with a stability factor slightly less than 1.0 may produce round holes in target paper, they are not stable. Projectiles with a stability factor only slightly greater than 1.0, while stable, are only marginally so. In practice, stability factor should be greater than 1.3, and 1.5 is better.
