Things to consider when choosing a caliber: Is it commonly available? If so, this will mean that a lot of people will want it, and therefore it will be valuable for trade, but also, a lot of people will have it, and it will be easier to get. In a Mad Max scenario, ammo will become a form of money, and oddball rounds will have no value, and will be harder to resupply.  The rounds on their own? Not a whole lot, which is why you'll be able to get a virtual monopoly on them whenever they're found.    The rounds plus a lot of spare rifles that shoot it? Congratulations, you have secured a perfect position for yourself post-SHTF. That guy you just handed a Steyr M95 and 20 8x56R rounds to, will have to bring you plenty of goodies to coax the next 20 rounds out of you, won't he? After all, you can't just get the ammo for it from a tree somewhere...