Abstract: An apparatus for the activation of a gas-operated reloading mechanism of a firearm comprising a cylindrical cartridge magazine housing along with a pressure sensing and regulating means, using a series of three chambers and a moveable piston which regulates a pressure controlling valve. Gas pressure from an exploding cartridge in the barrel of the firearm is bled into a first chamber through a passage where it displaces a piston attached to a moveable tapered stem. The tapered stem protrudes into an orifice creating a valve combination which regulates the flow of gas into a chamber which activates the reloading mechanism. High or low pressures within the gun barrel are equally capable of operating the reloading mechanism with reliability and without damage to parts.