The results of experiments with the initiator of lithium and graphite, the mass of which was ~ 10% by weight of the projectile, are presented. Such initiators create plasma with a high sound velocity value and provide the initial projectile speed of ~0.5 km/s. The drop-like damages on the insulating wall of the launcher barrel are described. There is a link between the appearance of these damages and sudden braking of the plasma piston. A hypothesis is discussed that relates the occurrence of these damages to the resonant deformation of the channel under the action of a moving high-pressure front. It is shown that the hypothesis of the resonant deformation of the barrel at a qualitative level explains the known features of the launcher operation.