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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Their rifle probably has a higher aim assist stat than yours, on top of the fact that its already easy as all hell. Try out rifle with higher aim assist; Shadow of Veils / Prudence II / Subtle Nudge / low Grade Humility / Etc. (Hidden hand perk works wonders.) Coming from a Battlefield 3 background, I'm accustomed to more skillful sniping mechanics.