I went to the range yesterday and when I arrived three guys were all hovering over a Bushmaster that wouldn't run. While I was setting up at the other end of the bench I kept looking over at the group to see what was up. I watched one of them take the Carrier assembly out a few times. Each time they would put it back in close up the rifle and try charging. I could tell that charging handle was stopping dead at about half it's normal travel.
Finally I walked over and offered my assistance. I pulled out the bolt and saw the problem right away. The carrier key was mounted backwards. I told them, "This won't work".
The real funny part was that the rifle was just sold. Both the new owner and the former owner were trying to figure out what was wrong along with the help of another guy. I guess prior to the sale the former owner did quite the job detail cleaning the bolt carrier assembly.