Without a doubt, the 22LR is my top choice for "survival" duties.
S&W makes a nice little AR style package called the M&P 15-22 which utilizes a 25 round magazine; also the venerable Ruger 10-22 is available. Sure, my Colt AR-15 and others will work, but without ammo they are useless other than clubs. And that is where the 22LR rules, I can carry 1,500 rounds if needed with relative ease, and combined with an optic they are deadly on all types of game, including headshots on deer and pigs, sitting birds, etc. Nothing else comes close to that kind of accuracy and firepower easily carried in a pack. Oh, and as a bonus the M&P 15-22 comes with a threaded barrel for suppressor.
The other great survival choice is my AK-47; nearly unstoppable survival weapon, slugs off dirt, sand, mud, extremely reliable and uses the powerful 762 x 39 round which I have in hollow points, soft points, etc. Only drawback is the weight. But it's a great SHTF rifle, mainly because of the standard 30 round magazines and knock-down power of the round.