This application discloses a firearm gas system that includes a gas block that defines a gas port and a passage that receives a gas tube and an orifice plate that defines at least two orifices, where the orifice plate is positioned between a gas port in the barrel and the gas port on the gas block and where the orifice plate is movable with respect to the gas ports in the barrel and gas block to selectively position one of the orifices between the gas ports in the barrel and gas block where the orifices in the orifice plate have different orifice sizes.