What I wonder is with this opportunity open for a very large national retail store to jump in on the vacuum that Walmart is leaving, who could do it? The largest retail stores on par with Walmart that have a big customer base would be Dollar General, but they've never had a sporting goods section and I doubt they'd be interested in the social media firestorm.
Lowe's and Home Depot could, but like Dollar General ammo and sporting goods is outside their scope of business. Ace Hardware is a big chain and I've seen basic ammo at Ace's out in the Midwest, but offering the amount of ammo Walmart does would be a big policy change for them nationwide.
So, the final possible option I can think of who I think would actually be interested in doing it, and I'm not joking, is Harbor Freight.
They have over 1000 stores in the US, they sell gun equipment already (ammo cans, gun cases, safes, brass cleaning tumblers, ultrasonic cleaners) and I will bet that a very large portion of their customer base are gun owners.
The only thing is I don't think people would be interested in buying Pittsburgh brand 9mm ammo with a lifetime warranty, so HF would have to break a rule and sell brand name ammo (Winchester, Federal, etc.) after buying from a distributor.