Complete the following tasks to unlock the the corresponding powerful weapon.
Old Gun (aka Canon Gun):
Beat the final boss.
Dragon Flamer (Upgraded Version of Flamer):
Complete the game on any difficulty setting.
Spell Gun (aka 'Sheep Mutating' Gun):
Complete the game.
+20% EXP Multiplier:
Purchase RIP 2 game on their developer website.
Double Barreled Pistols:
Complete the game, go to mission 2 and destroy some computer.