CO2 Gas BB Guns allow the BB Gun to be fired without the need to cock the gun before each pellet is fired.  Most CO2 Gas BB Guns are semi-automatic.   i.e  when you depress the trigger, one pellet is fired.  However, you may find some fully automatic models of this type too.   CO2 Gas BB Guns are generally slightly more powerful than spring powered BB Guns, making them very powerful indeed.  They use little compressed CO2 cartridges as the propellant and are slightly more expensive to run because of this, but they are very reliable.  The CO2 cartridge is installed into the gun and usually provides enough propellant for approximately 50 BB pellets to be discharged before needing to be replaced.