Dead Dust

As you're going through the game focus on picking up guns to use. The tommy gun and gattling gun will help get this quicker. You should get this before completing the game

Achievement Walkthrough:

After you finished the game, if you do not unlock this achievement yet, simply start a new game, pick the 1st character, you have 20 bullets when you start, just keep shooting and look for chest to see if there's bullet, if not, simple press start and quit to main menu and start again. You will be given 20 bullets again. I keep restart and after few minutes the achievement unlocked.