A pretensioner assembly for motor vehicle belt restraint systems is provided. The pretensioner assembly includes a tube in fluid communication with a gas generator. An elastic piston disposed within the tube is operable to drive a plurality of balls upon receiving pressurized gas discharged from the gas generator. The piston includes a proximal end and a distal end. The proximal end includes a flexible rim that expands upon receiving pressurized gas from the gas generator, and the distal end includes a flexible rim that expands in response to backpressure from the plurality of balls. The expansion of the flexible rims at the proximal and distal ends of the piston provides a tight seal between the piston and the inner peripheral wall of the tube.