the screw you are referring to is the ejector. when you retract the bolt after firing a round the extractor hooks onto the far right portion of the case rim and as the bolt travels back the rim catches on the tip of that screw protruding through the reciever, flipping the shell out and allowing the extractor to disengage the casing.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
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