I consider myself a decent sniper, but when I go into Crucible, I'm wrecked beyond belief. There are guys who headshot me the very second they scope in. It's -blam!-ing ridiculous! I don't understand how they do it!
EDIT: A lot of people are saying that they get their skill from Halo. However, I actually snipe very well in Halo, but not-so-well in Destiny.
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Muscle memory. Get used to common snipe spots and head glitches on the map. Majority of the time, my "quick scope" kill is really just a prediction based on radar, map knowledge, and the kill feed. Ultimate guide: http://youtu.be/qO7-Ylbgnig