Walmart claims to provide 20% of the National ammo sales. If this is true, then they will feel the lost sales.
Only as a brief dip, if that. They will be happily selling something else when the stocks of ammo run out. Remember they're not in business to sell ammo, or even sporting good, they're in business to sell things. And if the profit is there, one thing is as good as another, to them.
If you plan to boycott WalMart over this, by all means, do. But remember that you or I boycotting a store means NOTHING unless the store knows we are doing it, and why, and how much it is costing them.
In other words, for a boycott to have any chance of working, at all, you have to tell the company WHY you aren't shopping there, and how much they are losing. Otherwise, they won't know.