(extractor hang up). Where as the bolt is riding to far back in recoil and ejection of the fired round and creating grooves in the housing from the locking lugs and to the point as to sticking in that spot.
These are what I first train people on, Airsoft is not even close.. These are real guns that only can shoot brass blanks and they kick and go boom! http://www.maxsell.com/replica-guns/front-firing-replica-guns