From what I am told this is showing up in AR15/M4 type weapon systems that use piston drive gas systems. Example: HK 416s. This is effecting weapons that us the bolt carrier system that the AR15/M4 systems use. Basicly what is happening, that after a certain round count, 8K to 10K, the steel bolt carrier is wearing out the rear of the lighter alloy metals used in the upper receiver. What they believe is causing this to happen is that the gas piston is pushing on the bolt from the top of the bolt carrier, which is located to the front of the bolt carrier. As the bolt moves to the rear the pressure is forcing the bottom rear of the bolt carrier downward and because the metal is not as hard as the carrier, the bolt carrier begins to eat away the metal at the rear of the receiver. I have also heard that there has been a number of broken bolts associated with this as well. Has anyone ever heard, seen or experienced this?