A lot of guys who quickscope out of nowhere are running final round. Half of sniping is just about positioning and reducing your aiming area down to a small manageable area, preferably only needing small adjustments in the horizontal dimension. Skill plays a part, but skilled players know how to stack the odds in their favor. That's part of their skill. Also sometimes you just get lucky. I know sometimes I'll headshot a scrub flying through the air or something. Also you have to sort of have a sense for the center of your screen and be able to place targets there. Practice basically..