Finish Assignment Ada (available after completing the main game once)and you earn the option to buy this incredible weapon from the merchant in the main game. It costs a cool 1,000,000 though but is truly worth it for the insane overkill. Perfect weapon for speed runs.

Finish all 4 Mercenaries mini-game boards (available after completing the main game once) with a 5-Star ranking with ALL 5 unlockable characters to earn the Handcannon. Free from the merchant no less!