An injection system includes a syringe, which preferably includes larger and smaller inner diameters, and a plunger. A wiper sidewall of the plunger defines a cavity, into which a plunger shaft of the system is inserted, for moving the plunger, within the syringe, for fluid injection. The plunger includes a feature for engaging the shaft, and the sidewall preferably includes a expandable-contractible portion, in which the feature is formed. When the plunger is initially mounted within the larger inner diameter of the syringe, the portion of the sidewall is expanded and the feature of the plunger does not operably engage the inserted shaft. When the shaft moves the plunger into the smaller inner diameter of the syringe, the portion of the sidewall contracts and the feature operably engages the shaft. An optional deformable end wall of the plunger may be spaced apart from a distal terminal end of the inserted shaft.