Yaw of Repose
The small steady tilt a spinning bullet holds relative to its flight path, the aerodynamic cause of spin drift at long range.
A spinning bullet does not point its nose perfectly along the curving line of flight. As the trajectory arcs downward, the gyroscopically stabilized nose lags and settles into a small, steady angle to the airflow, and that angle is the yaw of repose. For a right-hand twist the nose holds slightly to the right, so the side force it generates pushes the bullet that way throughout the rest of its flight.
This tiny tilt is the root cause of spin drift, the lateral walk that grows with range and time of flight. The size of the yaw angle depends on the bullet’s stability factor and the rate at which the trajectory bends, and it is one of the small effects an external ballistics solver accounts for once distances stretch past several hundred yards. Understanding it begins with gyroscopic precession, the mechanism that makes a spinning nose respond sideways rather than straight on.