It depends on the model, but I know for a fact that on the AUG, it's a separate part from the hopup unit entirely.
http://www.arniesairsoft.co.uk/artic...m/aug/aug1.jpg
It's the tube in the middle of the ST-20 group.
The nozzle I'm talking about is the part on the gun that actually engages with the magazine, where the BBs are fed through.