Thanks. Any particular recommendations for boat guns in those calibers? Bearing in mind that its primary purpose would be general self defense in a marine environment, including against pirates?
7.62x51: FN-FAL (lots and lots and lots of these around), AR-10, Steyr AUG (expensive), modernized M14s and clones (except for the Norinco/PolyTechs), Galil, H&K G3 or 91 (later development of FN-FAL type).
Most of the M14 types are available with stainless barrels and actions, as well as synthetic stocks. 7.62 is also ideal for ventilating hulls and letting water in. That said, pirates have reportedly taken to using "unsinkable" types like Boston Whalers. In that case, 7.62 will quite happily penetrate the foam-filled hull and damage engines and fuel tanks.