Bullet of Bullets
While waiting for BoB to begin, Kirito assumes that Death Gun will sign into the tournament under a different name, given the perfect stage for him, and vows to find him and face him again. After much persistence, Kirito is able to convince Sinon to explain to him the details of the BoB royale. After she explains the rules and regulations, Kirito stops Sinon from leaving and asks her whether there were BoB participants that she did not know about. He then finds out that, aside from him, there are three other players who had not participated in the previous tournaments. He then explains her his motivation behind that question, and she theorizes from his explanation that he is an SAO survivor. Later on, during the tournament, Sinon decides to target Dyne and Pale Rider. Having found a sniping position on the west side of the bridge that Dyne had crossed, she prepares to take action, but is stopped by Kirito, who instructs her to not shoot and just watch Dyne and Pale Rider's battle. After Pale Rider defeats Dyne, he is suddenly shot with a silenced stun bullet and, after several moments, the shooter, Death Gun, reveals himself from the bridge. As Death Gun prepares to finish off Pale Rider, Kirito yells at a confused Sinon to shoot him before he does.
Aired 23/08/2014EP 8 - Bullet of Bullets

Sword Art Online II
One year after the SAO incident, Kirito is approached by Seijiro Kikuoka from Japan's Ministry of Internal Affairs and Communications Department "VR Division" with a rather peculiar request.
That was an investigation on the "Death Gun" incident that occurred in the gun and steel filled VRMMO called Gun Gale Online (GGO). "Players who are shot by a mysterious avatar with a jet black gun lose their lives even in the real world..." Failing to turn down Kikuoka's bizarre request, Kirito logs in to GGO even though he is not completely convinced that the virtual world could physically affect the real world.
Kirito wanders in an unfamiliar world in order to gain any clues about the "Death Gun." Then, a female sniper named Sinon who owns a gigantic "Hecate II" rifle extends Kirito a helping hand. With Sinon's help, Kirito decides to enter the "Bullet of Bullets," a large tournament to choose the most powerful gunner within the realm of GGO, in hopes to become the target of the "Death Gun" and make direct contact with the mysterious avatar.
(Source: Crunchyroll)