The areas you can save signifigant amounts of weight are in the barrel and stock. Sub 7lbs is doable. You might consider a Cavalry Arms lower which is polymer and included the stock. A light profile "pencil" barrel is a big weight savings. I would think a flattop upper with a MBUS rear would be about as light as possible. Stuff like the grip and charging handle probably aren't going to make enough difference to matter.