Injector devices and methods for using them to deliver medicament into a patient's body, e.g., sub-retinally within an eye, are provided. The injector includes a cartridge including a piston slidably disposed within an interior for delivering medicament within the interior through an outlet, and a driver including a plunger coupled to the piston, a source of pressurized fluid, and an actuator member for opening a flow path at least partially between the source and the plunger to advance the plunger and piston to deliver medicament from the interior. One or more sensors are operatively coupled to the plunger to measure displacement of the plunger, a processor is coupled to the sensor(s) for analyzing sensor signals to measure volume of medicament delivered from the interior, and an output device, e.g., one or more LEDs or speakers, provide outputs related to the volume of medicament delivered.