CO2 is the most popular air source on the market for paintball guns. Most paintball fields and retail goods fill CO2 tanks. A CO2 tank is categorized by the max amount of CO2 it holds in ounces (oz). Tanks are filled from reserve tanks in the form of liquid CO2, hence the 'oz' measurement. CO2 coverts from a liquid to a gas to propel a paintball. Heat is used in the liquid to gas conversion. As a result, CO2 tanks will become cold during use.