The stock motor on WE aeg models can be hit and miss depending on the version and a host of other things. As ghost said, just check the housing...you could also compare the motors and see if you notice anything.
Off topic:
Heh that must have been a fun time having your loaded m4 suddenly go full auto while you fumble to get the battery out. Guy with a highcap mp5 at a game had this happen....BB's EVERYWHERE. Im pretty sure he got some kills tho
