Eat Lead: The Return of Matt Hazard

The dual 6-shooters are found initially from killing bandits starting level 2, about half-way through. You can either pick up the weapons and kill a few, then die, kill a few more, die, etc... until the achievement unlocks, but that is not necessary - the bandits (cowboy looking guys) are throughout the game. By the time you reach the end, you'll be able to easily earn this one.