To attempt this, you must have at least three-hundred Medal Points (since this isn't actually a true "unlimited ammo" cheat). When you do, press Start and head to the Rewards menu. From there, head to "Weapon Select" and choose from the weapons available. This will allow you to choose any weapon with full ammunition once selected. Then, all you have to do is go back and forth to this menu, selecting and deselecting the same weapon to constantly refill your ammunition.