My 'survival rifle' is the NEF Handi-Rifle, "Survivor" model, in .45 colt/.410 shotshell. It has a rifled barrel for .45 colt, and a removable 'choke tube' for use with the shotshells, which is designed to 'un-spin' the shot load, which is spinning up to that point due to the rifling. You take the 'choke tube' out for shooting .45 colt loads. In the storage buttstock of mine, I have 24 rounds of .410 shotshells (12 2.5" 6 shot, and 12 3.0" 4-shot), and 6 rounds of 250 gr HP .45 colt.
http://www.hr1871.com/firearms/index.php?cat=6&subcat=5 (scroll down past the .223/.308 models)
As far as O/U combo guns go, you've got the Savage:
http://www.savagearms.com/24f20.htm
http://www.savagearms.com/24f12.htm
and what I think is the Baikal, soon to be imported by RAAC - now called the "Volk":
http://www.raacfirearms.com/Volk.htm
Remington imports the Baikal shotguns (single shot, O/Us, and S/Ss), calling them "Spartans", but apparently RAAC will be bringing in the combo guns. It's really too bad that RAAC is apparently not going to import the full line of Baikals like EAA used to, because they are really nice guns, and for the money, super nice.
and I'm sure a few others, in addition to the discontinued M6 scout.
For me, the Handi rifle Survivor is the best (for a food-getting-focused gun, not a defense and food-getting combo), because with shotshells, you can take gamebirds & small mammals, which are abundant (#6 shot for small birds & rodents, #4 shot for turkeys, ducks, geese, and large rodents like beavers). You can use the .45 colt or .410 slugs for large game like deer or pigs. And both the gun and ammo are a lot smaller & lighter than the combo guns. My second choice would be a plain ol' .22, like my Marlin 60, or something in .22 mag perhaps.
Wow, something just hit me upside the head, so to speak: Now I'm thinking that perhaps an argument could be made that the 'ultimate' survival rifle for BOTH food and self-defense, would be a semi-auto in .22 WMR (.22 mag), because it's plenty powerful enough to be a effective self-defense tool (obviously not ideal, but...), AND to take large game with, such as deer or hogs (both functions would require head shots and an accurate rifle), the ammo is still quite small & light, and a semi-auto would make for some firepower to keep the heads down of your imagined enemy (marauding islamo-rioters?). Hrmmm, what's a high-capacity, non-Ruger, semi-auto in .22 mag? My only .22 mag is my CZ 452 Varmint (turnbolt). Anything larger than .22 mag (like .22 hornet & up) would destroy way too much meat on your staple diet items (turkeys, rabbits, squirrels, ducks, etc.). And anything smaller (.22 lr) seems much too marginal even for head shots, on deer or a determined 2-legged enemy. A CNS shot from a .22 mag however, would be pretty reliable in stopping a 100-200 pound target.
Still, I'd much rather have several tools, not just one, for such a SHTF scenario, since the shotgun of some flavor is much easier to get moving game with, and something more powerful is desired for self defense. Still, it's interesting to ask, what if you could only grab ONE gun on the way outta dodge? Of course, if you're outta dodge, I doubt the islamo-fascists are going to riot much in the woods, so it's kinda silly isn't it?