There was one for sale on freeadds £50No, they were air guns. There's a Wikipedia article about this. Here's what it says:
"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."
I had one myself in about 1947, made with aluminium and completely useless.
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