A retrofit apparatus is disclosed for a gas-operated reloading firearm having a pressure control unit to operate the firearm in either manual or semi-automatic operation. The pressure control unit is coupled to the reloading unit to reduce a gas flow to the reloading unit in a first configuration and allow the gas flow to the reloading unit in a second configuration. A controller is coupled to the pressure control unit to operate the firearm in the first or second configuration. The firearm operates in a first configuration requiring manual operation until it receives continuous manual actuation to enable the reloading unit in a second configuration.