I have been asked and pondered this very question many times. In preparation for a lifetime stay in the northern wilderness of Alaska or anywhere in the whole world my choice for the most durable, weather and salt water resistant rifle ever devised is the discontinued but still available Ruger M77 MKII Stainless Steel All Weather rifle. also known as the canoe paddle. There might be a pin or two and some springs that are not a high grade of stainless. Given enough time all stainless firearms will rust so to improve it's weather resistance I would Cerakote every part inside and out with Cerakote except the barrel bore.
I would also add if I could only have one firearm to last me to the end of time with no tools or spare parts it would still be this Ruger. I would get it in 30-06 so the bore would be big enough to improvise a cleaning rod and doesn't build so much pressure that it is going to have much in the way of carbon or copper fouling that would stop up the rifle. The rings are the very toughest ring for any rifle. They need lapped but once done they are good to go. I would want a Leupold fixed 4 or 6 power scope and the factory iron sights should I ever break the scope. To take care of the bore I would render some animal fat and filter it and refine it best I could into a sort of grease to keep water off the bore.
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