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Another version of the origin story of the saying son of a gun goes like this:

When the settlers came to America from from Plymouth, England the trip took so long some of the women aboard had babies during the voyage. When there was an especially difficult birth the captain would order a cannon shot off on the deck above the birthing room. Thus the expression, "He's a real son of a gun."

I heard this from Ken Babbs one of the Merry Pranksters.