Oh! And don't ever get a rifle chambered in 5.56 "because in the case of the apocalypse, everybody else has the same thing, so ammo will be everywhere!"

... If you find ammo laying around, you know what else is probably within the vicinity? A firearm chambered in that caliber. No one is going to take a dead guy's AR-15, but leave all of his ammo for the next guy to grab. If you scavenge ammo, you're also scavenging guns, so no need to get the most popular offering just because it's compatible. There will be no shortage of compatible weapon systems.