Gus-
The problems I've seen have been bolts that are out of spec, usually in that the face of the bolt is too far back from the forward edge of the locking lugs. I've also seen firing pin holes that were too large combined with a tail on the bolt that was too short, and the pin would protrude too far and get stuck when the hammer fell, locking up the gun. Luckily we found that problem while dry-firing a new gun, as it could easily have led to a pierced primer.
My experience with my own Bushmaster products has been outstanding, but I've seen a lot of rifles have out-of-spec bolt problems. I've seen more of this on the LEO carbines than anything else.