I have recently purchased a Bushmaster XM15-E2S from a local FFL dealer. I am having some issues with it missfeeding and locking after emptying the magazine. This is what I have done so far. 1. i've tried replacing the magazines from the standard military issue mags to a PMAG. Still doing it. 2. I've tightened the magazine catch. Still doing it. 3. I've checked the bolt gas rings to insure they are 120degree offset from each other. Still doing it. 4. I've lubed the bolt and bolt carrier and all inside correctly. Still doing it. I allowed one of the knowledgeable FFl dealers to inspect it and they said it might be the ramp on the star chamber is to rough and needs to be polished. Nearest gunsmith wont be in till next week. any other ideas on what could it be before i spend money on it getting fixed?