Thin metal foils are used as quick-opening valves in gas guns to accelerate projectiles to speeds greater than 50 m s^-1. The mode and speed of opening are analysed. Measurements show that copper foils open in approximately 250 mu s. This value is in good agreement with a calculated upper bound of 280 mu s, which is the time of inertial rotation of the diaphragm. Foil rupturing does not appear to control the opening time.