Most airsoft gas guns are power by a variety of pressured gasses known as "Green Gas." Green gas in actuality is propane, mixed in with liquid silicon oil. This mixture helps promote the life of your gas guns' seal as using propane alone would dry up the rubber o-rings of your gas guns causing gas leaks. Green gas are not commonly sold outside of airsoft retailers and can be expensive, so airsoft propane adapters were developed to allow you to turn any 16 oz propane tank/cylinders into fuel sources for your airsoft gas guns. Drop a few drips of silicon oil regulatory and often, screw on the adapator, and fill away.