An adjustable gas block for a gas operated firearm offering a means to adjust a volume of gas directed to the host rifle's operating system. The adjustable gas block provides two positions of adjustment that are selectable based on the presence of a pressure increasing device being mounted on the firearm. A first position provides an optimal volume of gas into the gas operating system absent a back pressure increasing device. A second position usable in the presence of a back pressure increasing device causing venting of a portion of the volume of gas into a secondary chamber prior to release into the atmosphere. The amount of gas received by the operating system due to venting is reduced to a volume that is substantially equal to the volume of gas that would be received by the operating system absent a back pressure increasing device.