Six, I'm not sure about how to answer this one, but let me bounce this one off you. If the trigger housing does not seat properly in relation with the reciever, hasn't there been cases where the bolt will deal the hammer a "glancing blow" and not allow the auto sear to engage the hooks on the back of the hammer. Another senerio is that some bedding or other materials are in contact with the right hand side of the automatic sear that would not allow the sear to contact with the hooks on the back side of the hammer. However, in firing the weapon, the action is rough enough to dislodge the auto sear, causing it to hook the hammer. Just my .02 worth...Mike