A screw for injection molding provides a coaxial piston that allows the effective cross-sectional area of the screw to be varied during the injection cycle permitting small shot metering with relatively large diameter injection molding screws. The screw is positioned within the injector barrel of a injection molding assembly and includes a number of threads disposed around its outer surface configured to advance molten plastic to the front of the barrel. The screw further includes a central bore having a piston slidably received within the central bore. The plastic shot to be forced out through the nozzle of the injection molding assembly is controlled by movement of the piston within the screw.