Carbon Dioxide is used as a propellant in paintball.  A paintball Co2 Cylinder (tank) is filled with a certain amount of Liquid Co2 (measured in Ounces).  The liquid CO2 expands into gas state which is what a paintball gun uses to project paintballs.  When a paintball gun is fired, a small amount of CO2 is released which pushes the paintball out of the muzzle of the paintball gun.

Liquid CO2 can cause a lot of problems if it enters into the gun, including sporadic pressure variations.