An improved two-stage light gas gun for launching projectiles at high speeds. The gun consists of three tubes: the expansion, pump, and launch tubes. The expansion tube contains a close-fitting expansion piston that is propelled by an explosive charge. The expansion piston in turn drives the pump piston housed within the pump tube by means of a rod connecting the two pistons. The action of the pump piston adiabatically compresses and heats a light gas of hydrogen or helium, bursting a diaphragm at a predetermined pressure and expelling the projectile from the launch tube at a very high speed.