Any BOB must have ammo already in it. You don't need an "immense stockpile" if you're on the move. How many gunfights do you think you're to survive before you get shot? And you especially shouldn't count on scrounging ammo when on the move.
People can base their planning game plans on whatever they choose. My basis is this: if TSHTF, I can expect NO ammo to be available from outside sources. The only ammo/food/water/etc I can count on are what's in my house to begin with. From that, it follows that I want to minimize money spent and space taken up, and thus I consolidate on platforms and calibers. I expect only self-sufficiency. I definitely do not bet the ranch on scrounging ammo or food or anything. YMMV.