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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For pvp try an ambush scope. Relatively low zoom compared to the others but most maps don't call for any more zoom then that. Plus it's usually pretty stable so you can follow up with a kill shot if you miss the first one.