I'd have two separate answers, since the thread title says for 'shtf' and the OP says "teotwawki"; and those are very different things to me. Shtf to me is Katrina, LA riots, etc; a temporary thing. Teotwawki is The Postman, Mad Max, etc; a relatively permanent condition.
Shtf, my normal guns used now on duty and defensively - a CAR-15 with winchester's discontinued PowerPoint-Plus 64-grain load (I'm down to 15-16 boxes left now...
![](/images/smilies/icon_smile_angry.gif)
); and glock 19, usually loaded with federal 9bp-le 115 +P+ load. They're simple, reliable, effective. I've used the AR platform since 1981 and trust it completely, and used glock 9mm's since 1999, with the same trust now.
Teotwawki, though, I'd change my choices. Long-term scavenging/foraging resupply would make me want guns that lend themselves better to more primitive/simple reloading & casting; and the bullet casting rules out the 5.56 imo; for me personally at least. My true "Daniel Boone" lifestyle combo would be a levergun & revolver combo as others have mentioned above, but I'd go with my ruger srh .454 which has had its barrel shortened to 5", and the stainless Puma levergun to go with it. Power levels from 300 ft/lbs to 2,000 with the handgun, and more than 3,000 ft/lbs with the rifle; all with no adapters, chamber inserts, separate magazines, etc. The large caliber lends itself to decent power levels with unpatched & un-checked bullets, and it can even be effective using black powder, which many cartridge guns really don't.
Honestly, I'd almost be as comfortable carrying just the revolver without a rifle. One unique factor of the srh among .454 revolvers is that it allows speedloader use - both HKS and safariland speedloaders can be had that work in (I have some of both), and they work fine. That speedloader capability, plus the ability to use ammunition with such a broad power-range make it a suprprisingly capable defensive choice. If it were truly "teotwawki", my "one gun" choice would absolutely be that ruger.
{edited to add pics - any excuse for gun porn...
}
Handgun & levergun combo in .454 casull. Ruger with barrel shortened to 5" in home-made crossdraw holster is what I now carry in the woods with reduced (.44-mag level) loads. Speedloader carrier for the revolver, and drop-down 12-round carrier for topping off either gun. The levergun can be loaded/unloaded directly at the tube much like a .22 rifle, which eliminates jacking every round thru the chamber to unload at night:
http://i51.photobucket.com/albums/f360/quake316/LE/454combo-edited.jpg
Ruger srh, using hks speedloade; as I've had folks online claim there's no way they work. Been using them for a number of years now:
http://i51.photobucket.com/albums/f360/quake316/LE/SRH-HKS-2-edited.jpg
Rifle now carries ten rounds on a sling I had made up for it - six full-power casull, and four .45LC handloads:
http://i51.photobucket.com/albums/f360/quake316/LE/Photo0913.jpg