It has its own Wiki page!
This forward-moving barrel has given rise to the view that the Gat is not an air weapon at all, but simply a spring catapult.
This is incorrect.
The piston is a leather seal around the outside of the inner barrel,
running in a larger concentric cylinder within the receiver.
The barrel and piston are driven forward when fired,
air passing into the chamber from two small transfer ports drilled at the rear of the barrel,
just ahead of the leather seal.