[deep in the bush]

Originally Posted by lupo

Dirty barrel , hopup bucking , mags.
Defective/non-functioning hopup , mag , mag catch , nozzle , tappet plate.
Poorly installed hopup , tappet plate mag catch.
Wrong nozzle or hopup installed.
Missing hopup springs (pushes hopup back into gearbox on some aeg's)
Gearbox alignment - Gearbox is misaligned or is too big/small for proper alignment.
I think that covers it but I could have missed something. There are alot of factors with missfeeding and fps spikes. Alot depends on the model/brand of aeg when determining the cause.

double feeding can be loose piston too. Great list man, I hope this helps the OP.