If you have plenty of money and have unlocked a certain gun, you can buy that gun and sell it for a very reduced price. However, when you sell the gun, any ammunition inside it when being sold is transferred to your inventory, allowing you to "buy" ammo, albeit at a very high price Note: Buy the lowest cost type of gun because the ammo is universal. For example, it does not matter which pistol to buy, as long as you are trying to get pistol ammo.