If a bullet flies stable (gyroscopically and dynamically!) and
the transient yaw has been damped out, usually after a travelling distance
of a few thousands of calibres, the bulletīs axis of symmetry and the tangent
to the trajectory deviate by a small angle, which is said to be the yaw
of repose 
.
For bullets fired with right-handed twist, the longitudinal axis points to the right and a little bit upward with respect to the direction of flight, leading to a side drift to the right. The yaw of repose, although normally measuring only fractions of a degree, is the reason for the side deviation of spin-stabilized bullets.
