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Snipers are well known for their slightly sardonic sense of humour
The story of the 'Bootneck' sniper who had been inserted into his hide with rifle he was using for the first time.As luck would have it he was able to shoot a terrorist armed with a gun-the terrorist had walked round the wrong corner at the wrong time.
The sniper called up for a team to get him out.When they arrived,they walked towards where the body of the terrorist still lay on the side of the road,a bullet wound over one eye,'good shot said one of the Marines.The sniper merely grunted,took a closer look at the body and readjusted his sights.For a facing shot,snipers aim at a point between the eyes.He'd been out by two inches.
Aye

JR,
I expect the range and the target's own movements may have played a role in his disappointment.

There have been successful snipes at over 2km. Flight time of the round - about 6 seconds. How can a sniper accurately predict where a human target will be in six seconds time? There has got to be an element of luck involved.

Obi Wans right! There is an element of luck involved in accuratley estimating the position of a mobile target 4 seconds after you've pulled the trigger. I think the longest confirmed kill was by a Canadian called Carl Furlong at circa 2500y with a McMillan Tac.50 (not a Barrett, contrary to popular belief). In my opinion an awesome shot, I can only speculate what it is like to shoot at over 2k as the furthest I've ever shot is 1200y and that was a hell of a long way. This was done a civilian range (Stickledown-Bisley).
With regards to sniping I can only comment on the marksmanship aspect. You will need a great deal of patience and need to be reasonabley intelligent. Estimating range in different lighting conditions, weather conditions is alot to take in and the weather can throw in some weird phenomana of a bullets flight path too.
Lets put it this way to shoot at long distance nearly everything effects a bullet in flight, temperature, humidity, air density, wind, angle to target and at extreme range even the curviture of the earth (corrolis effect). This is after mastering things like stance, trigger technique, breathing, follow through among other things. So there is alot to take in.
It's also worth mentioning I've never shot the same rifle as the Royal Marines use or the same calibre (.338) so can't really comment on how the rifles shoot or how the round performs in terms of trajectory or ability to buck the wind etc. Although AI's are known for their excellent performance which is usually the industry standard .5MOA at 100y. Biggest round I've shot is .308 amongst others. Hope this helps.