Lucky you, I've figured out my choice!
Keep in mind, the situation where I would most likely be in survival mode is in the mountain areas. Not like the desert areas where there is nothing in sight for miles. With that being said my personal preference was to not choose a big game rifle or large pistol caliber because I don't feel like they are as adapt to as many certain situations as others.
Coming in as my 3rd choice is the .22 LR caliber.
Who doesn't love to shoot a .22? If you can find some ammo there's not much a .22 can't do in the right hands. Especially in survival mode. With good shot placement and a little bit of luck you can bring down big game as well as small game accurately. You won't be weighed down too much either carrying a lot of bullets for your trusty .22 firearm. I've shot coyotes and foxes well over 100 yards with a .22 Long Rifle while holding high above where I wanted the bullet to land and dropped them effectively. I've also shot many a grouse, squirrels, and killed lots of trees with a .22. It's not too good for fishing though. Keep reading and you'll understand. The main reasons I ruled out the .22 is because on a charging large predator it's not the most effective and not the best for shooting flying targets.
Coming in as my 2nd choice is the 20 gauge shotgun shell.
Why was this my 2nd choice? Because if I can't have a 12 gauge I might as well have a 20 gauge.
My personal #1 Choice would be the 12 gauge shotgun shell.
Maybe more specifically I just like the 12 gauge shotgun because it is so versatile. A 12 gauge can be a trusty companion for predator defense by throwing in some heavy shot or slugs but also just as effective for small game, flying game, and even fish with the right shot shell. How do you use a shotgun for fishing. One time when out hunting ducks on the Teton River with a high school buddy we were walking down the river bank when he spotted a fish not too far away and blasted at the water. To my amazement although the steel shot didn't make it down to the fish the force of the blast knocked the fish unconscious and it floated right up to the top.
While carrying around enough shells for 12 gauge can get a little heavy especially in a survival situation, if you're around any kind of game the 12 gauge can do the job in my opinion. You can still reach out and touch the further big game with a rifled slug in a smooth bore and dispatch some flying critters with some smaller shot while not having to be Annie Oakley or Deadeye Dick. You can be pretty accurate still with shot placement on a 12 gauge without destroying the game your shooting at.
If you don't have a slug with you and your only carrying bird shot you can still jimmy rig a shell to shoot like a slug. We were shooting clay pigeons for an activity once with the men in our church group and one friend showed me how to do it. Not that I recommend trying this because I've heard it will swell and expand your barrel as it travels out and can destroy your shotgun. Safety first :) He proceeded to take out his pocket knife and cut a thin line around the outside of the shell, at the base of the plastic wad tucked away inside the shell separating the powder from the shot so when the gun fired it would send a missile of everything intact above where he cut the line. He did this a few times and I was shocked to see that it penetrated a large solid tree stump a good 3-5 inches.
What I'm saying is, if you're ever on the show Ultimate Survival Alaska don't throw out the 12 gauge!