1. The most likely paths to global devastation (nukes, bio-weapons, The Jonas Brothers, etc ...) would cripple agriculture, making finding or growing plant food difficult or impossible in the quantities necessary to survive.
2. With no ability to manufacture or machine, guns will only be an effective means of hunting so long as the bullet supply lasts. And seeing as how the world will have recently been destroyed in a hail of gunfire, chances are that ship will have already sailed. This is doubly a problem as large scale conflict will decimate large animals as much as humans making the available stock left to hunt smaller than is practical to hunt with firearms.
So how will humanity survive? The answer: darts.
They're small. They're sharp. They don't need fancy manufacturing or supplemental materials. They can double as skewers to roast things you've caught over a tire-fire. You'll be able to easily make them from the bones of society scattered around you and easily pack them around as a nomad moving from region to region with changing of the nuclear seasons. I submit that darts are the ultimate tool for survival when the apocalypse comes. So long as you know how to use them.
And that's why tonight, you should swing by VFW Post 63, 3308 W. Chinden Blvd,. for a weekly meeting of The Boise Dart League. There you can hone your bulls-eye skills, your triple-20 skills, the underhand toss, the behind the back, etc ... You can learn to be an effin' dart ninja the tiny feral mammals of future-land will tell their children stories about to frighten them into eating all their depleted uranium.