Infinite Ammo mode is a special mode. When used, it gives Infinite Ammo to a clip of weapons, thus making the weapon capable of firing forever. Unfortunately, Infinite Ammo mode reduces Credit Earnings by 15%. Infinite Ammo mode is considered to be even more useful than Infinite Clips because it does not need to go through the withdraw reloading of clips that Infinite Clips has. I'm sure you'll have fun with your Holy Grail in Rapid Fire.