A firearm including a barrel, receiver, indirect gas system and an adjustable gas block. Four positions of adjustment are provided for on the adjustable gas block, each of which provides a different setting to regulate gas flow into the host firearm's operating system. A position of adjustment is selected based on how the firearm is being used. The provided positions of adjustment are calibrated for the standard use of the host firearm, use of a silencer, use of the firearm under hard or adverse conditions or when it is necessary to increase the firearm's rate of fire, and to block the flow of gas into the operating system. The provided gas block is designed to function with an indirect gas operating system, but could be adapted for use with a direct gas impingement system as well. Excess gas from the operating system is not vented from the gas block thereby eliminating the generation of additional muzzle flash and sound. No tool is required to manipulate the adjustment mechanism of the gas block.