CO2 is the oldest method of powering paintball guns. CO2 bottles hold liquid CO2 in them that converts to gas. This process keeps the pressure fairly consistent so long as the temperature does not change. Each tank is rated with a weight. This weight is the amount of CO2 the bottle can 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.