This gun paper model is a Barrett M82 (standardized by the US Military as the M107), a recoil-operated, semi-automatic anti-materiel rifle developed by the American Barrett Firearms Manufacturing company, this papercraft is created by Vov4ik Zver.
Designated as 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 BMG (12.7×99mm NATO) 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. More information at wikipedia.