Just in case what? Are they planning to eat it or try to sell it to gunowners at excessive prices when the SHTF? I don't think so...my 15 rounds in the mag ends the negotiations, period.
Not all would be doing it for that reason, at least not me. I would be doing it for protection. Guns are expensive, not everyone has one, but the ammo can be used by others in the neighborhood to protect the neigborhood, as well as barter for things they need that neigbors have.