Stability Factor
Also: Sg · Gyroscopic Stability
A number describing how well a barrel's spin stabilizes a bullet, where roughly 1.4 or higher is the accepted target for reliable flight.
The stability factor, often written Sg, quantifies how thoroughly the spin imparted by the rifling keeps a bullet point-forward in flight. It depends mostly on the twist rate paired with the bullet’s length, weight, and velocity. A value below about 1.0 means the bullet tumbles, while shooters generally want 1.4 or higher to keep flight clean across conditions and altitudes.
A healthy stability factor pays off as the bullet slows. Marginally stabilized bullets are most vulnerable in the transonic band, so a higher Sg helps the bullet stay pointed through that turbulent slowdown. Adequate spin also produces predictable spin drift, and a few specialized barrels use a gain twist to build stability gradually down the bore.