Interesting article on D.Report about two Brits "practicing their shooting skills" on the battlefied. Two with one shot or they refer to it as a "Quigley" shot. Caliber is 7.62mm NATO at ranges beyond 1000yds. That is getting out there for the 7.62 NATO round, but it can be done.

Here's one getting out there even further with 308:

Longest confirmed kill by a .308 rifle was 1,250 meters (1367 yards, 0.78 miles). Shot was taken by Staff Sgt Jim Gilliland of the US Army. It was one shot kill with a .308 M24 Rifle.