Within 40 days, the two marksmen from 4 Rifles, part of the Welsh Guards Battle group, had achieved 75 confirmed kills with 31 attributed to Potter and 44 to Osmond. Each kill was chalked up as a little stick man on the beam above the firing position in their camouflaged sangar beside the base gate – a stick man with no head denoting a target eliminated with a shot to the skull.

Osmond, 25, was an engaging, fast-talking enthusiast, eager to display his encyclopedic knowledge of every specification and capability of his equipment. He had stubbornly remained a rifleman because he feared that being promoted might lead to his being taken away from sniping, a job he loved and lived for. Potter, 30, was more laid back, projecting a calm professionalism and quiet confidence in the value of what he did.

Potter had notched up seven confirmed kills in Bara in 2007 and 2008 while Osmond’s total was 23. Both were members of the Green Jackets team that won the 2006 British Army Sniper Championships.

On one occasion they killed eight Taliban in two hours, ‘I wasn’t comfortable with it at first,’ said Osmond, ‘you start wondering is it really necessary?’ But the reaction of the locals soon persuaded him. ‘We had people coming up to us afterwards, not scared to talk to us. They felt they were being protected’.

Most of the kills were at a range of 1,200 metres using the 7.62 mm L96 sniper rifle.

The snipers used suppressors, reducing the sound of the muzzle blast. Although a ballistic crack could be heard, it was almost impossible to work out where the shot was coming from. With the bullet travelling at three times the speed of sound, a victim was unlikely to hear anything before he died.

Walkie-talkie messages revealed that the Taliban thought they were being hit from helicopters. The longest-range shot taken was when Potter killed an insurgent at 1,430 metres away. But the most celebrated shot of their tour was by Osmond at a range of just 196 metres.

On September 12th, a known Taliban commander appeared on the back of a motorcycle with a passenger riding pillion. There was a British patrol in the village of Gorup-e Shesh Kalay and under the rules of engagement, the walkie-talkie the Taliban pair were carrying was designated a hostile act. As they drove off, Osmond fired warning shots with his pistol and then picked up his L96, the same weapon – serial number 0166 – he had used in Iraq and on the butt of which he had written, ‘I love u 0166’.

Taking deliberate aim, he fired a single shot. The bike tumbled and both men fell onto the road and lay there motionless. When the British patrol returned, they checked the men and confirmed they were both dead, with large holes through their heads.

The 7.62 mm bullet Osmond had fired had passed through the heads of both men. He had achieved the rare feat of ‘one shot, two kills’ known in the sniping business as ‘a Quigley’. The term comes from the 1990 film Quigley Down Under in which the hero, played by Tom Selleck, uses an old Sharps rifle to devastating effect.

Potter and Osmond’s working day would begin around 7 am and end a dozen or so hours later at last light. Up to about 900 metres, they would aim at an insurgent’s head, beyond that at the chest.

Often, Potter would take one side of a compound and Osmond the other. Any insurgent moving from one side to the other was liable to be shot by the second sniper if the first had not already got him. Each used the scopes on the rifles to spot for the other man, identifying targets with nicknames to do with their appearance.

A fighter wearing light blue was dubbed ‘the Virgin Mary’ and one clad in what looked like sackcloth was referred to as ‘Hesco man’, after the colour of the base’s Hesco barriers. Both the Virgin Mary and Hesco man were killed.

Others were given a nickname because of their activities, like Hashish man, a Taliban who doubled up as a drug dealer. Occasionally, insurgents got posthumous monikers. If one target presented himself, both snipers aimed at him simultaneously in a coordinated shoot.

“Everybody you hit they drop in a different way,’ says Potter. ‘We did a co-ord shoot on to the one bloke and he just looked like he just fell through a trap door. So we called him Trapdoor Man.”

Major Mark Gidlow-Jackson, their company commander, describes Potter and Osmond as the “epitome of the thinking riflemen” that his regiment sought to produce. “They know the consequences of what they’re doing and they are very measured men. They are both highly dedicated to the art of sniping. They’re both quiet, softly spoken, utterly charming, two of the nicest men in the company, if the most dangerous.”

Serjeant Potter and Rifleman Osmond are identified by pseudonyms for security reasons.

Dead Men Risen, published by Quercus Publishing at £18.99 RRP, is available from Telegraph Books at £14.99 + £1.25 p&p. Call 0844 871 1515 or visit books.telegraph.co.uk