If you had to pick one gun, only one, for survival, what would it be?
I think I would choose a 12 Ga. pump, probably a Remington 870. With a supply of field loads with 6's or 7 1/2's, some OO Buck, and some good old 1 oz. rifled slugs I think you could handle a lot of different situations from defense to feeding you and your family. Finding ammo along the way probably would not be too hard either.
Only one? Are we talking SHTF street survival, or lost in the woods survival?
SHTF street survival would be my Mini-14 with a good supply of ammo.
Lost in the woods, If I knew ahead of time I was going to be lost in the woods, I would take along a shotgun with a variety of ammo, for taking birds, deer, pigs, whatever. But then again if I had my wife along to tell me which way to turn, I never would have gotten lost in the first place.