There are high quality 4k textures applied to the separate standard PBR shader for each LOD model. Textures are easy to downscale.
Every LOD model has albedo, metallic (with smoothness channel), normal and ambient occlusion maps.
For effective culling each model is presented as a group of multiple meshes (rifle, magazine, special 12.7x55 mm cartridges, shell, grips, switches) with a single material. All parts are movable.