This is the Sniper Rifle System 99 Anti-Matériel was used by the UNSC Army during the Fall of Reach. It's not 100% LEGO, but it's close enough. A dowel rod was used to support the barrel and custom waterslide decals were applied to its exterior. Overall, the riffle is is 5'3" (1.60m) long, and apparently, very heavy.
It features a sliding bolt, a removable magazine, and a trigger that can really be pulled. Read an in-depth feature on the riffle at The Brothers Brick.