It has happened more than a few times, where the upper receiver basically shears apart and blows out the sides. Had a buddy who was standing at the range one day next to a guy who had this happen. Luckily the shards in my buddies neck and arms weren't too small and could be pulled out easy enough.
Btw... if I can find a picture of the injuries, and also of the gun on the net, I'll pm it to you. It was a pretty top end AR as well to note. Started going in FA and I guess a round didn't chamber right, and basically detonated the casing from the absolute chop of the bad FCG. Either way I wasn't there to witness it, so I guess it's only as so true as I trust my good buddy. But either way, things tend to always be exaggerated in talk, but those pics were def scary though.

