I have modified two mosins to extract the [too long] 300WM and 7mmRM cartridges. I call it the "ejector override button".
I also modified the magazines to fit the wider shoulders.

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The button is on the left side of the receiver.

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There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. -Ernest Hemingway
The man who makes no mistakes does not usually make anything.-- Edward John Phelps