While the current record for the longest “confirmed” sniper kill is over 2,700 yards (just over 1.5) miles, with a .338 Lapua Magnum round, for nearly 35 years the record stood not with a traditional centerfire rifle, but with a modified M2 Browning Heavy Machine Gun, affectionately known as the “Ma Duece”
The “sniper?” None other than the famous US Marine Gunnery Sergeant Carlos Hathcock III. Using a 8x scope common for the era (a Unertl), Hathcock engaged the Vietcong from a stabilized tripod mounted M2. Per the book Inside the Crosshairs: Snipers in Veitnam, by Colonel Michael Lee Lannig stated that Hathcock engaged a moving target on a bicycle at over 2,500 yards, requiring two shots.
Hathcock’s first round disabled the bicycle (blowing off the handlebars), the second struck the enemy soldier in the chest.
The record stood for over 35 years including US missions in various low-intensity conflicts such as Somalia, Panama, etc., and larger wars including Desert Storm. It was finally broken in 2002 by a Canadian sniper using the same round, albeit this time in a purpose-built highly-accurate platform using likewise purpose-built ammunition.
Gunnery Sergeant Hathcock is still a legend within and outside the Marine Corps. Post-Vietnam, Hathcock helped established the Marine Corp’s Scout Sniper school along with training the best of Tier 1 units including Navy SEALS, Army Delta, and others.