Precision weapon

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The Sniper Rifle, a precision weapon, in action during a match of Firefight.

"Particularly useful against delicate equipment—like brains."
Halo: Reach manual description[1]

In the context of the Halo games, a precision weapon is any weapon which is capable of inflicting a headshot upon an enemy. Save for certain boss enemies such as Hunters and the Warden Eternal, scoring a headshot on an unshielded enemy with a precision weapon will instantly kill them, regardless of how much health they had remaining.