Interesting.

Perhaps the survival purposes for carrying firearms are important in examining redundancy and backup. The two survival purposes that seem most discussed are self-defense and hunting.

If self-defense is the goal, then a most-likely-scenario analysis would seem appropriate. Somebody other than me will have to address that.

I can easily think of situations where a pistol, a shotgun, or a rifle might be the most appropriate self-defense primary or backup weapon. The major alternative would seem to be to avoid conflict, which might not always be a choice you can make.

If hunting is the goal, a most-likely-scenario analysis also seems appropriate. Others have addressed this many times, and seem to conclude that nets, traps, and snares are more efficient than firearms for most game with the possible exception of animals larger than deer.

If that conclusion is correct, then a rifle or shotgun of sufficient caliber for such larger game would seem to be the gun of choice.

Firearm candidates that combine both self-defense and hunting would seem to include either rifles or shotguns, with the edge toward shotguns for the variety of shot, slugs, and powder a single shotgun can use. If longer-range use figures as a most-likely-scenario, then combo shotgun-rifle firearms would seem a powerful option.

Redundancy seems a big challenge given the expense, weight, and size of a single rifle or shotgun. Again, with significant weight penalties, a combination rifle-shotgun seems to include some built-in redundancy.

At the moment my brain cannot puzzle through the backup challenge.