I just wanted to report a bug where the Perfectionist perk won't activate for the Event Horizon sniper rifle. Here's a clip of me getting the three necessary headshots and still having to reload:
http://xboxclips.com/Sneak+Attack+65/fcb1e4b0-949c-4523-aaef-d34d76a21138
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-blam!-. And I thought it was a God roll.