I think this movie is loosely based on a real person/event.
I just watched a documentary on real military snipers on the history channel. They mentioned a sniper called Carlos Hathcock who was one of the best snipers in history. The way they described him and his missions resembled Thomas Beckett in the movie Sniper and his missions. Example: Carlos Hathcock shot another enemy sniper who was stalking him through the scope and killed him. Thomas Beckett did the same to the enemy sniper who was stalking him. There was also mention of a mission that Hathcock went on that was very very top secret to kill a high level military offical and it had him crawling through high grass on this stomach for 3 days to the compound where he would make his shot. After he took his shot it raised all hell and everyone went after him. This is exactly what happened at the end of the movie Sniper too.
There are other similarities mentioned in the documentary as well but it really seem too much of a coincidience if it wasn't loosely based on this real life sniper. Anyone know for sure?