If you're not at the store when the stock in question gets unloaded and pushed to shelves at Wal-mart (most of the 'semi-pro' resellers know the timing and are lined up at 7-7:30 AM when stuff is pushed to the floor) or know how to get your allocation peeled off the shipment before it hits the shelves at Fleet Farm you are out of luck.
The website in question is using data provided by the Wal-Mart mobile web app and is generally accurate for the day a product is on the truck from a DC to a store, however I seem to recall it's slow at indicating when product has sold-out which is almost always the case the minute it moves out of the stockroom and into the hands of the folks waiting in line to grab their daily ammo ration.