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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A lot of experienced sniper really are just that good, but keep in mind that on occasion the lag/latency will also allow enimy players a little extra time to bead in on you before they actually show up on your screen.