High velocity guns, very very high

A little known type of gun once dominated the high velocity gun world. It was called the "light gas gun" (LGG). Outwardly resembling a large, conventional gun or small cannon, the LGG used a novel means of attaining higher velocities than usually possible with gunpowder.

Normally, powder guns are limited to muzzle velocities of about 5300 feet per second. This is because gunpowder can only burn so fast and cannot push a bullet any faster. At some point adding more powder only acts like a hevier bullet, as the powder must push its own weight too.

The LGG has a chamber divided into two separate sections. The rear section contained the gunpowder. The forward section was filled with a low density gas such as helium. Dividing the forward and rear chamber sections was a movable piston. In front of the piston was the helium. In front of the helium, holding it in, was a thin disk. In front of the thick disk was the bullet.

When the gun was fired, the powder burned, building up pressure which pushed the piston forward. As the piston moved forward it compressed the helium. At some point the helium was under so much pressure that it would rupture the disk and push the bullet down the barrel. Since the helium was so light, most of the energy could go to pushing the bullet. Because the pressure of the helium at disk rupture was so great, the bullet achieved a phenomenal muzzle velocity, perhaps as much as 14,000-17,000 feet per second.