Well, there is one sure thing, that nearly every Wal-Mart that sells ammunition will carry 223, 22-250, 243, 25-06, 6.5 CM, 270, 7MM RM, 30-30, 308, 30-06, and 300 WM, so if you don't reload or find yourself in a pinch, you can probably find ammo within a reasonable distance from nearly anywhere in the contiguous 48 States.
I don't recall ever having seen a single box of 257 Roberts or 6.5x55 factory ammo at Wal-Mart and it is less common, with less variety, than any of the 11 cartridges listed above at the 3 biggest retailers that carry ammo near me, Cabela's/BPS, Guns Unlimited, and Scheel's.
