this is exactly why i limited my calibers last year, of the handguns i own i have enough ammo to keep me going for a while if i got to pack up and go i don't have several stashes to put with individual guns. a select few calibers and grab the guns and go.
point 2 if I carry 1000rounds of 9mm i will be carrying half the weight of you 45acp guys meaning i can almost double the stash I carry when setting out on foot 115 vs 240 grain.
now as far as availability, don't judge by what you can find on the shelf, in a SHTF scenario there will be no ammo on any shelf, you will be relying on what you can scavenge on the streets.
FYI.....1,000 rnds of 9MM 115 grn = approx 30lbs 1,000 rnds of 45ACP 230grn = approx 50lbs
I don't know about you but I'm not a pack mule so the thought of humping that much weight in just ammo makes my back and legs hurt just sitting here. If I'm on the streets it'll be light and fast.