The decelerating mechanisms and existence conditions for the pinch structures obtained in plasma rail guns are considered. The plasma is confined by the magnetic field of the self-current. It is shown that under experimental conditions the predominant effect is induction deceleration of the pinches by plasma formed behind the pinches as a result of ionization of vaporized electrode and accelerator wall material. The expressions obtained for the limiting velocities and pinch radius are found to be in satisfactory agreement with experiment.