Here's my WalMart online inventory experience:
Generally it's pretty accurate. There seems to be a 15-30 minute delay once they sell out before the website is updated. I know this because I've checked the online inventory, bought the rest that was on the shelf, then checked 15-30 minutes later and it has changed to out of stock. I've seen some stores listed as Limited Quantity, and for me that has meant they are out. I have also seen some items listed in stock for weeks that I know they don't have. The biggest indicator of whether they actually have stock is if an item went from "out of stock" to "available."
Now around me there is one WalMart in my town, 3 in the town 20 minutes north, and 3 in the town 40 minutes south. So when I check an item for my store and it has the "Find in another store" option I see a few different locations at once. For my store it seems that they get their shipment about a day behind the others if some ammo comes trickling in. If I can't make it to one of the stores north or south of me when they show stock, I check back later to see if my local store has it.
One issue is when you know they have stock and they don't. This usually happens when you notice something go from out of stock to available, but you check the ammo cabinet and it's not there. I found one evening that some 223 was listed as in stock after it had been out for weeks. Went into the store and found the spot on the shelf was empty. Asked one of the employees to scan it with their handheld and they said it was listed as "wharehouse out." He explained that it meant it was in transit to the store and they were expecting a truck that would probably be unloaded at around 3am (this was 10pm). I got up in the morning and went back and there was no one from sporting goods available to help me, but another employee scanned the tag again and said it was listed as on hand. But only a sporting good person can apprently retrieve it to stock the shelves and sell it. I had an appointment to go to so I left and came back later. Once I got back I asked the on duty sporting goods person and they said they had received some but that it had already sold out. Unfortunately that particular item has showed in stock ever since so I'm not sure if it will be replenished. I should have pushed to have a manager check on that one.
The three stores to the north also showed stock so I went up there and found that the first store did have stock and bought some, the second store had only a couple left so I bought them, and the third store had a grumpy old guy working. At the third store I couldn't find a sporting goods person so I asked someone in toys and they scanned the tag for me. They didn't say what the scanner told them, but that they would get someone to help me (seemed like a good sign to me that it was showing in stock). Unfortunately the guy just came over and said they hadn't had any for weeks...didn't even scan it even though I insisted. He said he had done the morning inventory, but from my experience I don't know that I trust that.
Next week I see the 3 stores to the north have some other ammo in stock again, so I head up and the first one they guy is just opening up the box to stock the shelf so I got a few there. The second store didn't have it out, but I asked the guy there and he found where they keep the ammo before it gets put out and was surprised to see that they actually did have it. He stocked the shelf and let me buy my 3 boxes, and thanked me for letting me know they had it in stock. Third store had already put theirs out and had sold a couple, but there was still some for me.
I've never had the e-mail alert feature let me know that ammo has come back in stock.