San Tan's STT-15 Pillar Billet Upper features truss pockets. The truss design acts similar to a bridge that carries and distributes the stress across the entire platform verses putting all of the stress on the material right behind the threads of the receiver. The extra strength gained from this design is not only to help the receiver. It also in turn helps improve your accuracy. Thru the use of 3D CAD modeling software, San Tan Tactical is able to save weight in places that extra material didn't have a purpose, such as the forward assist block and the shell deflector. NOTE: Requires a forward assist, charging handle, barrel, handguard, gas block, muzzle device, gas tube and BCG (Bolt Carrier Group) to complete.