A cartridge gas port selector mechanism for a firearm barrel having a plurality of gas ports extending from the barrel bore and having a gas block with gas passages in communication with a gas tube. A selector valve member is movable within the gas block between an open condition permitting flow of cartridge gas from one or more ports of the firearm barrel to the cartridge gas passage and a closed condition blocking flow of cartridge gas from the barrel bore to the cartridge gas passage. A valve selector or actuator is connected with the valve member and is selectively moveable to position the valve member at the open condition or the closed condition to select the volume and timing of cartridge gas communication from the barrel bore to the gas energized autoloading mechanism of the firearm.