In a SHTF situation when society has pretty much collapsed? Or when things are pretty much as they are now, just pretty darn crappy.

SHTF I'd bring an M4rgery with maybe 200-300 rounds. In an all out firefight, I'd not expect to get through more than a few magazines if I was very lucky, and stupid to get in a gun battle in the first place. But most of the ammo I'd trade out for food and mutual protection.

If I just lost my job and had to leave looking for work like the dustbowl era... then probably a simple 9mm concealed with a few spare magazines. Small enough to conceal well, big enough to be useful.