From Halopedia, the Halo wiki
Retrieved from "https://www.halopedia.org/index.php?title=Top_Shot&oldid=1278802"
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. The achievement only be unlocked on an Anniversary map.
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 an homage to this show, considering Top Shot was one of the sponsors for Halo Waypoint's Top 5 series.