Want to fill up your Discovery with CO2, but it's filled with air right now? Vice versa? If you don't want to dry-fire the gun to get rid of the air or CO2, you can quickly and safely remove all of it with this handy degasser tool.
Screw the tool into the hole in the back of the receiver until the degasser has made contact. Then, a couple more turns, and it'll start exhausting the air or CO2.