If you want to build an AR-15 under 5 pounds, you are going to have to get a little creative and it’s going to cost you. However, you don’t have to start skeletonizing things that shouldn’t be skeletonized or using exotic materials to build a rifle that will still carry light and function reliably.
SIONICS recently posted a picture of one of their Patrol Three Rifles on a scale. It weighs in at a scant 6.1 pounds with top quality, standard parts and in a ready to shoot configuration (with sights).
How do they do it? Did they flute the bolt carrier or use aluminum where you should use steel? Nope. This rifle still has a proper M16 BCG and other reliable, full weight parts. They just make use of a smart barrel profile and quality, lightweight hand guard. Those are the same places you should start when shedding weight. Or… Just check out a Patrol Three at SIONICS.