Get a five star rank with all mercenaries in the Mercenaries mini-game to unlock the HandCannon weapon. To get unlimited ammunition for it, fully upgrade all of the HandCannon's options. Alternately, successfully complete the game two times. The HandCannon will be available as soon as you start the game again for 0 Pesetas. It is a very powerful magnum, and will have unlimited ammunition. When you go to upgrade it in the merchant window, in the "HandCannon" update screen there is an option labeled "Exclusive". When chosen, the gun is upgraded with power identical to the Chicago Typewriter.