For me personally... I like to buy low so I just buy it when it is on sale and keep some on hand in case of a SHTF scenario. Boy Scout Motto... Be Prepared.
If the SHTF to the extreme... I figure 1-2k rounds of various .12ga, 9mm (what I currently own) would do me until things worked themselves out.
*edit* Oh and I have a thing about shelf life so I cycle my ammo and emergency supplies to make sure they don't get stale...