Issue with CO2 is once you put the bulb in you have to use it or you are wasting a what ever is left in the bulb. Also I really don't understand why some one buys something and then say what other manufacturers mags fit. Stick with like for like. You got a WE gun get WE mags you are not going to run into fitment/seal issues.

Looks like copper oxidisation.

price per shot with CO2 vs GG on a WE pistol is near identical, and actually works out cheaper if you buy the bulbs in packs of 40 or more. Only way to make gas cheaper is to be buying plumbers gas and using an adapter.