Refillable Gas BB Guns allow the BB Gun to be fired without the need to cock the gun before each pellet is fired.  Most Refillable Gas BB Guns (Compressed Air BB Guns) are semi-automatic.   i.e  when you depress the trigger, one pellet is fired.  However, you will find some fully automatic models of this type too.   Refillable Gas BB Guns are generally slightly more powerful than spring powered BB Guns, making them very powerful indeed.  They use bottled compressed gas as the propellant and are fairly cheap to run and very reliable.  The refill nozzle is a small hole, usually located at the butt of the gun, and looks similar to a refillable lighter nozzle connection.  When the gas chamber is full, the gun should have enough propellant for approximately 50 BB pellets to be discharged before needing to be refilled.