Ideal Choice
I don't really care what conventional wisdom says, a lever action carbine is far more accurate and powerful than any handgun and far faster and easier to use than full-length bolt-action rifles. They're the aurea mediocritas of firearms. People think of leverguns as archaic, but a .45 Colt or .44 Magnum lever action will allow you to totally dominate a 75 yard radius. If you can't do it with eleven rounds, it's not likely more are going to help. The real trick is to have buddies with weapons of their own.
I suppose it wouldn't be a good choice to go after soldiers armed with 400 yard rifles, but you're probably screwed no matter what if that's your scenario. IMHO, you're likely to want something fast, light, easy to handle, tough, and powerful. If it's the lever that's the problem, a semi-auto carbine would also work well. The more weapons I own and shoot, the more I'm convinced carbines are an ideal tactical weapon.
As far as carrying one, the backpack scabbard is great. I've used it in the field with everything from carbines to shotguns and heavy Mausers. Believe it or not, even the heaviest rifle FEELS LIGHTER than a revolver on my belt.
Spare ammo could go in the backpack, whether in bandoliers or not. That stuff does get heavy real fast, so I wouldn't carry more than fifty rounds plus a pocket press and some basic reloading tools, assuming you were off on some strange quest. Though I honestly don't know why you'd want to leave home if the SHTF. They always do it in movies, I suppose.