G23f Full auto Gas leak

[DJ AoS]

Hi guys,
This is probably a simple problem but I know nothing about GBB, defo an AEG man but I have a KSC G23f which I hardly use but now when I fire on Full Auto it just pisses gas out the back (you can feel it on your arm). It still fires just looks like its smoking (from the rear).

Is it a leaky mag / seal? I really dont know. I had a bit of a search but came up wth nothing.

Over to you.. :-)

Cheers
Alex