If you are a fan of lightweight AR-15 carbines, you likely remember the joint project between Battle Arms Development and Brentwood Gunsmithing called the OIP Carbine or Ounces is Pounds Carbine. Battle Arms Development released an image of one of the receiver sets with a very cool natural anodized finish. This is the best look at what it takes to make a sub-4 lb. AR-15 yet.
You can see that extra material has been aggressively milled away and even skeletonized in some places. The top rail has been shortened to allow just enough space for an Aimpoint Micro. The stock is a minimalist nub of aluminum. Every part was scrutinized and I suppose that is what it takes to make a carbine that is a full 2 pounds lighter than what most consider ultralight when it comes to AR-15s.