A while back I did some caliber consolidation, sold weapons and ammo of the lesser-needed varieties and kept mostly only what I consider to be in my primary weapons systems. I eliminated .40 S&W (ammo and four pistols), 7.62x39 (ammo and one rifle), 4.6x30 (ammo and one rifle), others that I can't even remember.
But back to the original question, if the world went to smoldering shit and I would be doing one-day scouting trips, I would probably bring 5.7x28mm along - I'd have my PS90, Five-seveN, and shorty AR, and a bunch of rounds. But if I were busting out with a doubt as to whether I'd ever make it back to home, it would be my standards of 22lr, 9mm, 45ACP and 5.56.