Wouldn't need a pistol for anything on that island. So to take advantage of the gun being free, I would like a Ruger Bisley (stainless) in 45 LC that had been improved by Bowen. NP3 on all the metal.
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I might add that when I leave home every day I am better equipped than Hanks was, I have a Spyderco & a cig lighter & at least one handgun. I have a BOB in the car. In winter I have my coat (gortex) with me everywhere that has a leatherman, matches, two flashlights (mini mag light and a surefire, have spare set of batteries for the maglight), firstaid kit, spaceblanket, gloves.
And I know how to make fish traps. Scurvy would be my biggest concern until I found something to eat that had Vit C (I haven't studied tropical survival much, I know what to eat in northern climates to avoid scurvy, but unless there were citrus fruits around I would have to guess blind on seaweed).