survival gun to me is defined IN PART by portability (weight, size) adaptability (different game different ranges), availability of ammo (223/308/.22), and flexibilty (change to different configurations, uses).
One firearm that I find lends it self greatly to each category (after a great deal of time, research, and trial in the field over many years) is the Contender platform. With a minimum of fuss, and a modicum of expense I can change in about 30 seconds from .22 to 223 to 308. I can carry all three calibers in 14 inch barrels, so I can reach from right up close to over 300 yds, with excellent accuracy.
With the .223 I can hit center of mass of a 180 lb + target every single time out to 400 yds. that kind of reach allow for engaging hostile targets at a far enough reach to make a change in location (fire and maneuver) feasible for a single fighter with limited resources.
it makes a great scout arm, and a superb hunter. Being at scout level, means never having long drawn out fire-fights, and practice makes you a rather rapid single shot firer. In fact after long years of practicing and teaching "Controlled fire" I just can't see a need for full auto (every wanna-be's wet dream, I know) on that level (survival/scout/hunting/foraging rather than combat infantryman in a platoon size or bigger unit).
I hunt extensively from late August to late fall, and do long range varminting in all months, especially in spring for ground squirrels (.22 at up to 100 yds) and Prairie dogs (223 up to 250 or even 300 yds) , as well as coyotes (300-350 Max Prefer to call in to 100 yd)).
If I had to leave camp suddenly, the contender would go with me, If I had to make a jump into the back country, the contender would go. When archery hunting in Oregon, I take the Contender in 7TCU for bear in a shoulder slung holster.
If this is a planned excursion, I'd take all three barrels, 100 rounds each, and plan on trapping, snaring, foraging, etc without shooting.
the three barrels weigh far less than any two guns.
Sudden notice? the contender resides next to the bug out bag, in a canvas bag with spare barrels, 100 rds each and cleaning kit.