How about "What will be the most common ammo to be found after TEOTWAWKI"?
Of course, that ammo might be found hiding out in someone's basement. Or on the body of some Bad Guy who just made the mistake of trying to accomplish a lone-hand-raid on your hidey-hole. Or under the detritus in the store-room of the local hardware store, after the rioters and looters have abandoned it.
Lots of possibilities, here.
THAT might regulate one's choice of what firearm to hold onto longest.
MY personal choice will be something like 22LR, 38SPL or 9MM.
LOTS of possibilities.