Barrett M82 .50 Caliber rifle
The M82 (also more recently known as the M107) is a recoil-operated, semi-automatic anti-materiel rifle developed by the American Barrett Firearms Manufacturing. A heavy SASR (Special Application Scoped Rifle), it is used by many units and armies around the world. It is also called the "Light Fifty" for its .50 caliber BMG (12.7 mm) chambering. The weapon is found in two variants — the original M82A1 (and A3) and the bullpup M82A2. The M82A2 is no longer manufactured, though the XM500 can be seen as its successor, in that it also employs a bullpup configuration.

From
http://en.wikipedia.org/wiki/Barrett_M82
http://www.inetres.com/gp/military/infantry/rifle/M107.html