Interior ballistics
The interior ballistics - a sub area of ballistics - comprises all processes in the shot development like gas pressure and bullet velocity from the chamber to the muzzle.
The term “free flight” describes the distance of the rotation-free bullet travel when the bullet is no longer in the case neck, however without having been pressed into the lands (rifled part in the barrel).
Grooves (recesses in the barrel) and lands give the bullet a stabilizing spin (rotation/twist) around its longitudinal axis.