Looking to get a PSA Gen 3 18" .308 upper to complete a build for a SHTF rifle that is under 10lbs and capable of up to 500 yard shots. Not sure if that is achievable. I can get the weight there with the bare rifle, but I also need an optic. A red dot will not be sufficient, so I need glass in at least 5x or 6x. Any ideas for a budget optic that is compact and lightweight?