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Perfect Gunner (20 )
How to unlock: Don't Miss With a Machine Gun.
To acquire a Machine Gun, you can do one of two things: kill the three convicts in Leisure Park and detach the Machine Gun on the back, or you can go to the gunstore on Day 3 and kill Brett Styles to get his Machine Gun. If you choose the latter, come prepared with Shotguns and Uzis, as he and his friends will fire on you and you will have to kill all of them if you intend to kill Brett.
After acquiring your Machine Gun, go to Leisure Park and carefully fire each round at a zombie, ensuring that each hits. This will take quite a while, but it will work and you will get the achievement.