CO2 is the oldest method of powering paintball guns. CO2 bottles hold liquid CO2 in them that converts to a gas to operate the paintball marker. The liquid and gas in the CO2 tank natually maintain a pressure of about 800psi at room temperature which will hold until all the liquid converts to gas or the termperature changes. Because of this 'self regulation' CO2 bottles do not have reguators, just pin valves.
CO2 Tank sizes are measured by the weight of CO2 they can safely hold in ounces. At 70 degrees, a 9 oz bottle will shoot about 350 paintballs, a 12 oz bottle 500, and a 20 oz bottle will shoot 900 paintballs. Some paintball guns are more efficient than others. Cold weather will decrease the number of shots you get. Firing your paintball gun quickly will also reduce the number of shots per fill. In cold weather, CO2 does not work very well. If you are playing in colder weather, fire the gun with longer pauses between each shot.