Expansion Chamber
A feature of some gas-blowback guns, this chamber is located between the gas reservoir and the nozzle. The gas is first released here to expand and create high pressure that is then released to do its job when the trigger is pulled. This means less gas will escape in liquid form, which would waste gas and may lead to freezing of certain key parts like O-rings. Note that an airsoft gun with an expansion chamber is capable of firing with the magazine removed if there is a BB in the barrel!