This was despite the 8.59mm bullets leaving the barrel of his rifle at almost three times the speed of sound.
The distance to his two targets was 8,120ft, or 1.54 miles – according to a GPS system – and about 3,000ft beyond the weapon’s effective range.
The 35-year-old beat the previous sniper kill record of 7,972ft, set by a Canadian soldier who shot dead an al Qaeda gunman in March 2002.
Speaking about the incident, Cpl of Horse Harrison said: “The first round hit a machine gunner in the stomach and killed him outright. He went straight down and didn’t move.
“The second insurgent grabbed the weapon and turned as my second shot hit him in the side. He went down, too. They were both dead.”
The serviceman then fired a third and final round to ensure the machine gun was out of action.
He said: “Conditions were perfect, no wind, mild weather, clear visibility. I rested the bipod of my weapon on a compound wall and aimed for the gunner firing the machine gun.”
He killed the two insurgents as he protected his troop commander, whose vehicle became trapped in a field in Helmand Province and started coming under fire.
Cpl of Horse Harrison, from Cheltenham, Gloucestershire, was using the British-built L115A3 Long Range Rifle, the army’s most powerful sniper weapon.
It is only designed to be effective at up to 4,921ft – just less than a mile – and capable of only ‘harassing fire’ beyond that range.
To compensate for the spin and drift of the bullets as they flew the length of 25 football pitches, Cpl of Horse
Harrison reportedly had to aim 6ft high and 20ins to the left.
In a remarkable tour of duty, he cheated death a few weeks later when a Taliban bullet pierced his helmet but was deflected away from his skull.
Source: Adam Arnold for Sky News.