Wow. This thread resurrected itself. I'm sure S&W will sort out the issue with your rifle. I've only had one instance of a cartridge getting jammed up like that, and not on my S&W. It happened on my home built AR. The best I could figure is that a lightly charged cartridge didn't provide enough gas to fully cycle the BCG. The short stroke caught the round while it was tilted and jammed it into a lug.

Drop mag, clear the action, triple confirmed the rifle was in a safe condition. Separated the upper from the lower, then shined a light down the bore from the muzzle end to check for a bore obstruction. No barrel obstruction. Reassemble & continued my range session.

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-John