A gun powered by compressed gas with no loss-making expansion space, having: a pressure chamber interconnected with a barrel, and a projectile opening of the barrel at an inlet of the pressure chamber, the pressure chamber and the barrel being mutually lockable by a valve, wherein upon firing the gun: the pressure chamber and the barrel form one space, the valve is connected to a tie rod, and passes through the pressure chamber to a striker space, wherein within the striker space: the tie rod is fitted with a spring, the striker and an end of the tie rod are equipped with a stop, and the spring is positioned in the striker space between the striker and a special section wall, and wherein: the barrel, tie rod, valve, and striker are mounted along one axis, and the valve is controlled by the striker via the tie rod.