Abstract: A gas-powered gun fires a projectile at a settable velocity unaffected by substantial increases in the operating pressure of a gas cartridge. The gun has a compartment for accumulating a charge of gas. The compartment includes a chamber whose inlet is controlled by a valve member normally in a closed state. A biasing spring urges a piston assembly within the chamber to deflect the valve member to an open state, allowing pressurized gas flow from the cartridge to the chamber. The pressure of the accumulating gas charge urges the piston assembly away from the valve member. The gas accumulated in the compartment attains only a predetermined threshold when the valve member returns to its closed state. A trigger-controlled discharge valve releases the accumulated gas charge at the threshold pressure into the barrel to fire the projectile.