weight and commonality are the biggest factors in what i invision, my picks are in order of importance.
.22LR, 40gr CCI Velocitor
- velocitor is probably the most powerful of LR out there, .22LR is always attractive in SHTF requiring high mobility, long term survival or threats like zombies. the packaging is helpful in keeping it clean and might make it more attractive for barter. If you have a silenced gun this might not be suitable.
.223 Remington, 62gr FMJ SS109 "green tip" or Natec Spectrum Polymer Cased Ammunition
- about the same weight as 9mm, easy to use from iron sights, useful in paramilitary oriented SHTF scenarios. The green tip has better penetration but if you can find it Natec is lighter and has less muzzle flash. If you're a really incredible shot and see the need for long range work go for a .308 instead.
9mm Parabellum, 115gr JHP CCI Blazer,
- cheap and lightweight, some people have problems with aluminum cased ammo, my beretta handles it fine. if you anticipate you'll have the ability to reload cartridge cases then 9mm isn't optimal anyway. Go for the JHP but if you really think you need extra penetration CCI does make a TMJ 147gr with aluminum case.
.410 Slugs, pretty useful and common, gets a nod especially if you're thinking about a SA M6 survival rifle.
all these cartridges are available in a good mix of guns, some very compact and that's again important for weight and that "provisional governments" might want to confiscate them. none of my picks are particularly manly, if you anticipate threats like Deathclaws and Supermutants adjust accordingly. I have to agree with 1911Tuner, handguns aren't very good in SHTF.