A BB gun with CO2 cartridges.
Propellant
In ordinary guns, the source of propellant is gunpowder, which, upon combustion, drives a projectile due to the pressure produced by the rapidly expanding gasses in the chamber. A CO2-powered gun uses a similar principle, except the pressure is created by a blast of gas from a compressed CO2 bottle that is fixed to the gun. Pulling the trigger opens the valve connecting the bottle to the gun, releasing the blast of compressed gas. The valve then closes until the trigger is pulled again.
CO2 Bottles
CO2 bottles contain carbon dioxide that is mixed with a small amount of oil and pressurized into liquid form. Because they contain liquid CO2, firing them causes the bottle to suddenly become cold, in much the same way that a CO2-based fire extinguisher turns cold quickly. Some of these bottles are the disposable powerlets, which were popular with CO2-powered guns in the 1990s. They have since fallen out of fashion and have been replaced in many CO2-powered guns with refillable bottles.
Refilling a CO2 Bottle
Many believe that the bottles from CO2-powered guns can be refilled simply by hooking them up to an air compressor. There are paintball and air guns that are operated that way, but these are called high-pressure air (HPA) guns, which are different from CO2 guns. Furthermore, regular air is not 100 percent--or even mostly--composed of carbon dioxide. Refilling a CO2 bottle requires that liquid CO2 be used, and this requires specialized equipment.
Gaseous CO2
Some CO2 guns use gaseous CO2 bottles instead of liquid. As gaseous CO2 is not as compressed as liquid CO2, it requires a bigger bottle to carry the same amount of propellant. A typical gaseous CO2 gun has a hose connection running to a bottle--mounted either on the back or strapped to combat webbing--whereas liquid CO2 bottles are small enough to be attached directly to the gun. The gaseous CO2 gun has the advantage of having a quicker "recharge rate," which is the time it takes for pressure to be fully restored in the bottle. The obvious disadvantage is having to carry multiple larger, heavier bottles.
Tags: CO2-powered guns, carbon dioxide
