There is nothing quite like hot soapy water IF you dry it completely afterwards and apply the right lubricant (in our case silicone oil).
This was one of the amazing things I had learned about Real metal blackpowder guns, and even real modern guns for that matter. It sounds like a totally dumb thing to do ("wont it rust?" comes to mind) but it does work.
If ever you buy a real rifle from WW1, WW2, or whatever has sat for a decade or two in packing grease... try it. It's even better if you can submerge everything except the wood parts (if any) in a deep enough sink-full of water and plain old dish soap. Then when it's all clean; dry and oil. The hot (not scalding) water will evaporate faster.
This is not something to do to most airsoft guns, but when it's as bad as you've just shown... there's a point where you got nothing much to lose.