G&G Mp5a5 strange problem.
 

So, my friend have that G&G mp5 a5, it works great overall, but at some point when he fire on full auto, after like, 10-50 shots(It's rather random between those number of shots) The Mp5 just stop shooting and he must press again on the trigger.

I sold it to him, I never had this problem before with it, mainly because I never used it more then a few time. Never really kept firing that much.

He wants me to repair it before paying me for it.

What I though it could be was probably the selector switch just stopping the contact after a certain time, or just moving as the gun lil recoil goes.

It's not a big deal IMO, but he wants me to fix it, note that it would be the 4th AEG I open. But I'd like to know if it's worth the time to open it or if it's not a big deal or not even a problem. Just so I can show him the page or something.

Edit : Apparently it's when he push the trigger all the way down.