This is a modified skin for the G36 in S.T.A.L.K.E.R. It now looks more like the weapon used by the german army.
Extract the files in your S.T.A.L.K.E.R. directory which for retail buyers should be located in C:/Program files/THQ/S.T.A.L.K.E.R. -Shadow of Chernobyl/Gamedata/ and for Steam users it should be located in C:Program files/Steam/SteamApps/Common/Stalker shadow of Chernobyl/gamedata