$9 PBR Aimpoint CS Sight Low-poly 3D model by steffe-engdahl. Available on CGTrader with 5116 polys and 2723 vertices. Check it out on CGHub.
OBJ (OBJ, filesize: 858 KB), FBX (Autodesk FBX, filesize: 186 KB), MA (Autodesk Maya, filesize: 507 KB), DAE (Collada, filesize: 789 KB)