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Generally speaking, if you fit the head of an NSF agent fully inside of those sliders (and even sticking out a bit) you'll stand a much better chance of getting a lethal headshot. If you wait a while, the sliders will go down a fair bit, and the bullet will appear anywhere within the edges of the sliders. Also, if you've ever played Rainbow Six, you'll note a similarity in the way that the targeting reticle works, namely the four sliders that determine how "accurate" a shot is going to be. The higher your pistol skill gets, the more damage you do per shot.
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