[Con Murder]

Last week a friend called me up cuz is new mp5 (G&G) had some trouble. Talking to him he said he tightened the but plate all the way and couldn't fire. Telling him how stupid he was I started to loosen the screw, since it should fire I was amazed when it shot twice then siezed. I opened it up and looked at the anti-reverse, couldn't trip it so I opened the mech box. Reset everything and tried again, it locked. So I tested his battery in my gun, and it was dead, so I opened the mech box (and got advice on how to trip the anti-reverse from the motor hole from ASC) and tried with a full battery. Same as last time it locked.

I decompressed the spring and set it aside for another day. Well the same thing keeps happening. I was so certain the spring was in backwards, but no I checked and the tighter coils are at the spring guide. I thought 'maybe gear timing?' then found out it is a myth.

What would you suggest?
Oh and if you want me to send it to a gun doc, I'll try a hundred more things first.