Which one?
The front baffle, which is right behind the locking lugs; is the primary seal in the event of a case-head failure.
The rear baffle, which is the moon shaped piece in front of the bolt handle; is the last seal in case of the aforementioned malady. If gases get into the bolt body, rather than flowing out the cocking slot into your face, this stops it. It also provides a mechanical starting point for the extraction of a fired case.
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