To be honest, the whole "post-apocalyptic" thing is pretty much a libertarian masturbatory fantasy anyway. Government completely collapses tomorrow, a fresh slate upon which the new order can be built. An interim where men are men, women are women and we all live in bunkers with our automatic weaponry, relying on our wits, selfishness and huge gold reserves to survive.
Because that's totally likely to happen. It's not like governments around the world have continuity plans which stretch right down to the individual town level, and that should something happen to cause central government to lose control, that local government, law enforcement, local military deployments and similar will step into the breach. And those plans are pretty damn detailed, and have quite extensive chains of command.
Eh, I could see the whole post-apocalyptic scenario happening in a very localized sense. I mean, small towns fall apart all the time even with a functioning federal infrastructure, for simple economic reasons. And then you have places like Detroit and Camden, New Jersey. But yeah, even then guns and bunkers would be incredibly unhelpful, and any inhabitants would likely die off at an alarming rate. About the closest I think we could get to the libertarian fantasy is a quasi-feudal state ruled by warlords and fundamentalists in the vein of Somalia. And unless they're willing to go out, organize(not likely) and raid farmers for their crops, these gun fetishists are ending up at the bottom of the shitheap. That's not even getting into the practical issue of whether there'd be enough unprotected farmland to survive on with banditry for a year, or how long it'd take law enforcement and military to start cleaning them up with overwhelming superiority in firepower, discipline, morale, numbers, and supplies.