I'm not sure I like that list.
I see three types of SHTF:
1) "Lost in the woods..." for whatever reason (disease, earthquake, massive storms, etc.) you are cut off from normal supplies and may not be high on anyone's priority list for a while. That's a fairly big risk if you live in the mountains or desert actually, because a storm or landslide can leave you stuck in the boonies with no support and nobody will notice for days. It is possible you'll need a firearm to defend against predators or fellow travelers with no moral code, or as a signaling device, or to shop for groceries. A hunting rifle would be perfect for this scenario.
2) "Temporary break-down of authority..." the big example around here would be the LA riots in the early 90s... the Police Chief actually had the cluelessness to get on TV and say "our biggest priority is the safety of our officers." ... of course, because of that cowardice and general mismanagement from the top down, there were days where the only pockets of law and order were those pockets created by store owners with rifles on top of their stores, scientologists with baseball bats standing around their buildings, and so on. People with courage and something to defend. In that scenario a .22LR would be just fine because defending property didn't mean leaving piles of bodies. There was no problem buying groceries (just drive to the local Super Wal-Mart), no problem staying safe (just drive past the local Super Wal-Mart), no loss of communications or anything... but if you had property in the area you had to defend or hire a private security guard because the cops are going to leave and hold press conferences until it all blows over.
3) "Total break-down of social order..." that's the one we haven't faced in a long time, but it has been faced in many places. In that case, your goal is to survive long enough to find an external support network (people in other nations who believe in your cause) and bring in real weapons through them. In Afghanistan that support network was first the US (during the Soviet years) and then various muslim groups (during the Northern Aliance years... including today). The last time the US really went through that was arguably the Revolution. Having a battle rifle would be nice, but rockets, explosives, and so on would be nicer. And you've got to ask yourself whether you are going to want to be a soldier in this new revolution whatever it is. Probably not my sort of thing.
A decent hunting rifle... a Garand being acceptable... will cover most of that.
engaging in that sort of foolishness would get you immediately killed from ten directions at once by trained people with superior equipment who operate in well-praticed squads.
Maybe we should send some of those well trained people to Iraq.
