Top Shot
From Halopedia, the Halo wiki
Top Shot
Top Shot is an achievement in Halo: Reach multiplayer matchmaking added with the Anniversary Map Pack. It is awarded for getting 3 headshot kills in a row in one matchmaking game on any of the Anniversary maps.[1] The achievement only be unlocked on an Anniversary map.
Trivia
On the History Channel there is a show called Top Shot. It is a competition where sixteen great marksmen are chosen and compete for the title of "Top Shot." This achievement is most likely a homage to this show, considering Top Shot was one of the sponsors for Halo Waypoint's Top 5 series.