To understand how BB guns work, let's first look at conventional
guns. The basic idea behind almost any firearm, from a pistol to
a cannon, is to create a small explosion just behind a
projectile positioned in a cylinder. When the explosion tries to
expand in a gun, it is very limited as to where it can go: The
edges of the cylinder are stronger than the force of expansion,
so it can't expand out in every direction. The only way it can
expand is down the cylinder, the barrel of the gun. If a
projectile (a bullet, for example) is in the way, the expanding
matter from the explosion will push the projectile forward as it
makes its way down the barrel.  The principle behind BB guns is
pretty much the same as in firearms, except there is no
explosion. The expanding matter is ordinary air, carbon dioxide
or some other gas. Before you fire the gun, the gas is
compressed so that it has a greater density, and consequently a
greater pressure than the air in the atmosphere outside the gun.
The compressed gas is stored i