Besides a mid-sized 9mm pistol, another hugely versatile firearm to have when things get bad will be a .357 Magnum revolver with a stainless steel finish and a 4 inch barrel. Such a gun can chamber and fire both .357 Magnum and .38 Special rounds, and the stainless steel will be very rust and corrosion resistant.
Why the Ruger GP100 in particular? The main reason is because it’s simply the most robust and durable .357 Magnum revolver on the market today, and it was designed specifically to fire an unlimited number of full power .357 Magnum rounds. In contrast to this, most other revolvers will overheat and the cylinder or trigger will lock up, and you will need to allow the weapon to cool down before you fire again.