On earth projectiles are accelerated to hypervelocity using two-stage light-gas guns. A diagram of a typical light-gas gun is shown below. The first stage is a larger diameter cylinder filled with compressed (50 psi) hydrogen gas. The breech contains a powder charge, while the other end of the cylinder, called the pump tube, is tapered. Inside the pump tube is a nylon piston; when the powder charge is ignited, the piston is propelled down the pump tube to the end of the first stage, compressing the hydrogen gas to extremely high pressures.
The second stage of the light-gas gun consists of the barrel, the flight range, and the target chamber, and is separated from the first stage by a scored rupture disc. The projectile is located at the front of the barrel, just downrange of the rupture disc. The pressure in the second stage, like space, is near vacuum. When the pressure in the first stage becomes sufficiently high, the rupture disc fails, releasing the hydrogen gas at hundreds of thousands of pounds of pressure into the vacuum of the second stage. The rapidly expanding hydrogen propels the projectile down the barrel to the target.
With the high pressure behind, and the vacuum in front, the projectile will reach velocities in the neighborhood of 7 km/s before striking the target.