The dynamical motion of a FSAPDS projectile is affected due to the propellant gas force, aerodynamic, gravity along with mechanical force, and shockwave forces. In the sabot opening process, the mechanical action takes place and the sabot gets separated due to the shockwave force. In external mechanical action of sabot separation, the maximum stretch of the spring defines the end of third phase of motion. In this paper the motion of the projectile and its stability during this phase has been presented. The time delay in the sabot opening affects the stability of the projectile is discussed with the help of modified stability parameter.