A needleless injector wand assembly comprising a tubular two-piece barrel including a front section and a rear section; a first chamber in a front, discharge end of said front section; a one-way valve on said front section for receiving an injectable liquid from a source thereof and feeding the liquid into said first chamber; a nozzle on a front end of said barrel for discharging liquid from the first chamber; a plunger slidable in the front barrel section for movement between a retracted position in which liquid is drawn into the first chamber and an extended position in which the liquid is discharged through said nozzle; a second chamber in said rear section of the barrel rearwardly of and separated from said first chamber; a piston assembly slidable in said second chamber for movement under fluid pressure between an extended position in which it moves the plunger to the extended position and a retracted position in which it moves the plunger to a retracted position; a third chamber in said barrel surrounding said second chamber for alternately receiving gas under pressure from said second chamber during movement of the piston assembly from the retracted to the extended position, and for discharging the gas under pressure to said second chamber during movement of said piston assembly from the extended to the retracted position; a breach plug closing a rear end of said rear section of the barrel and including a first passage for receiving a gas under pressure from a source thereof, and carrying the gas to a rear end of the piston assembly to cause the piston assembly to move from the retracted to the extended position, and a second passage for carrying the gas under pressure to and from said third chamber to cause said piston assembly to move alternately from the retracted to the extended position and from the extended to the retracted position; a magnet assembly (98, 99) on a rear end of said piston assembly for attracting the piston assembly to the breach plug to retain the piston assembly in the retraction position until the gas under pressure is introduced into said second chamber behind the piston assembly; and a pair of tubes (143, 144) connected to a rear end of said breach plug for carrying gas under pressure to and from said breach plug; characterized in that the rear section of the barrel includes an elongated tubular body connected to the front barrel section by a tubular plug the plug extending into the rear barrel section and containing said second chamber, the tubular body and the tubular plug defining said third chamber between the body and the plug and surrounding the second chamber formed within the tubular plug, and an opening in said tubular plug permitting the flow of gas between the second and the third chambers (111, 133). a tubular two-piece barrel including a front section and a rear section; a first chamber in a front, discharge end of said front section; a one-way valve on said front section for receiving an injectable liquid from a source thereof and feeding the liquid into said first chamber; a nozzle on a front end of said barrel for discharging liquid from the first chamber; a plunger slidable in the front barrel section for movement between a retracted position in which liquid is drawn into the first chamber and an extended position in which the liquid is discharged through said nozzle; a second chamber in said rear section of the barrel rearwardly of and separated from said first chamber; a piston assembly slidable in said second chamber for movement under fluid pressure between an extended position in which it moves the plunger to the extended position and a retracted position in which it moves the plunger to a retracted position; a third chamber in said barrel surrounding said second chamber for alternately receiving gas under pressure from said second chamber during movement of the piston assembly from the retracted to the extended position, and for discharging the gas under pressure to said second chamber during movement of said piston assembly from the extended to the retracted position; a breach plug closing a rear end of said rear section of the barrel and including a first passage for receiving a gas under pressure from a source thereof, and carrying the gas to a rear end of the piston assembly to cause the piston assembly to move from the retracted to the extended position, and a second passage for carrying the gas under pressure to and from said third chamber to cause said piston assembly to move alternately from the retracted to the extended position and from the extended to the retracted position; a magnet assembly (98, 99) on a rear end of said piston assembly for attracting the piston assembly to the breach plug to retain the piston assembly in the retraction position until the gas under pressure is introduced into said second chamber behind the piston assembly; and a pair of tubes (143, 144) connected to a rear end of said breach plug for carrying gas under pressure to and from said breach plug; characterized in that the rear section of the barrel includes an elongated tubular body connected to the front barrel section by a tubular plug the plug extending into the rear barrel section and containing said second chamber, the tubular body and the tubular plug defining said third chamber between the body and the plug and surrounding the second chamber formed within the tubular plug, and an opening in said tubular plug permitting the flow of gas between the second and the third chambers (111, 133).