Gyroscopic (static) stability factor
This figure shows the gyroscopic stability factor of the 7.62
x 51 Nato bullet M80, fired at an angle of departure of 32°, a muzzle velocity
of 870 m/s and a rifling pitch at the muzzle of 12 inches. The M80 bullet
shows static stability over the whole flight path as the static stability
condition sg>1 is fulfilled everywhere. The value of
sg adopts a minimum of 1.35 at the muzzle.
Generally it can be stated that if a bullet is statically stable at
the muzzle, it will be statically stable for the rest of its flight. This
can be easily understood from the fact, that the static stability factor
is proportional to the ratio of the bullet´s rotational and transversal
velocity (see formula ).
As the the rotational velocity is much less damped than the transversal
velocity (which is damped due to the action of the drag), the static stability
factor increases, at least for the major part of the trajectory.
Bullet and gun designers usually prefer sg > 1.2 ..1.5
at the muzzle, however it has been observed that many handgun bullet show
excessive static stability.
  
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