Folks often say that the longest confirmed sniper kill has been made with 338 Lapua. Without knowing its shooters' hit:miss ratio per shot fired, that's probably just some luck of the odds. May still be if the hit:miss ratio was known. We don't know how may shots he made at that distance and missed, do we? If any of us did that a lot of times on a lot of targets, we too would eventually make a hit with the first shot at one.
That shot was made by the late Chris Kyle.
No it was not. CPO Kyle made a 2100 yard shot in 2008 with the Lapua. The longest kill with a Lapua is 2,707 yards by a British sniper in 2009. The record is held by an Australian sniper with a .50 BMG, 3,079 yards, 2012. These are all confirmed kills. One other note, these guys don't usually luck into hits, they hit because they know what they're doing.