The overhead on those big stores is phenomenal. They put all the clothing out front, because the mark-up on clothing is typical 300-400 percent, and gives them their greatest profits. The mark-up on fishing equipment is similar. The mark-up for Guns is about 25%, and sometimes the discount places are forced to sell guns for even less of a mark-up. If they want to stay in business, they have to sell lots of clothing, and lots of fishing tackle, and gear.
The Cheapest way for them to sell guns, and make a profit, is to let you order the gun, paying for it in advance, and giving you a discount over regular retail price. With you paying in advance, the store invest NONE of its own cash to buy the gun, and still makes a small profit on the transaction.
The gun does not sit on display for months, eating up cash and making no money on the investment. As soon as your gun arrives, a phone call to you is made, and it leaves the store. The store has its "commission" for handling the paper work, sales, tax, Nics, check, etc. from the start, and that profit can be rolled over quickly by investing in more items that sell quickly at a much higher mark-up.
When you see a gunstore with lots of firearms on display, the owner has invested a lot of money to stock that store, and needs to be selling lots of guns every day to justify that huge investment in inventory.