I would like to spend some time to share how to fix the mag for VFC MP5, it works for me so far.
MP5 mag fix 1
first fill the mag with gas and check where exactly the leakage coming from, by hearing the leak and by smelling the gas to locate it.
in my VFC MP5 mags, this is the worst leakage coming from causing by the sealing o-ring doesn't fit in no matter how tight you screw it.
second, may be here?
third, may be from the intake valve? that could be complicated!!
Don't try to tighten the intake valve if there is no leak, leave it. Or you may possible unscrew part #47 Extend charge Tube inside the mag and cause major gas leaking.
this leaking from here is complicated too!!
If still having problem to locate it, put it iunder water to check it, you'll see bubble.