Practice. I suggest looking at TrueVangaurd's videos about Sniping.
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TrueVanguards sniping is ridiculous. It literally looks like he just waves the reticle over targets all shaky-like and gets headshots. In several instances you can actually pause his videos and see that the reticle is nowhere on the persons body or head when he takes some shots and still gets a headshot. Don't believe me? Just go watch his how to snipe videos and pause at the moment of firing. You'll see...
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Its called aim assist
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I know what aim assist is and I've pulled out some sniper rifles with high aim assist as well as rolling hidden hand for even higher aim assist. It was impossible for me to replicate that style of sniping even with high aim assist. I'm even using KBM. If my reticle isn't squarely on their face or neck, it's not giving me a headshot.