If your Mac is not ejecting the spent brass cases, it may be a worn extractor rod. As the primer is hit and the charge detonated, the bolt is hit and forced to the rear of the receiver. Once it hits the buffer, the extractor rod is pushed through the bolt assembly and along with the extractor, pops the spent case out. This rod is also know as the extractor rod. (Rod only, other parts sold separately) Please call us if you have any questions.