Abstract: An ignition tube for use with electrothermal chemical combustion ignition projectiles in guns, comprising a tube positionable in a combustion chamber for receiving plasma ignition from a plasma chamber. The tube is formed from high density polyethylene or other materials adapted to release plasma upon contact by plasma ignition electrical energy or pulses. The tube is specifically designed to provide an increasing exit area from the proximal end of the tube toward the distal so as to act upon the longitudinally attenuated plasma, thereby substantially decreasing the amplitude of any reflected shock in the plasma stream. The exit area comprises a plurality of radially extending orifices or holes that have a decreased angle of inclination to the longitudinal axis from proximal end to distal end, such that the orifices form a spiral pattern on the circumference of the tube. This pattern provides a uniform action of plasma on the propellant over the entire length of the ignition tube.