A two piece upper receiver for a firearm including a first piece having a front end, a rear end, a top, and a bottom, the first piece constructed to operatively carry a bolt carrier and bolt, and to have a barrel coupled to the front end. The bottom is constructed to have a lower receiver coupled thereto. The first piece includes a backbone extending along the top between the front end and the rear end, the back bone having upwardly directed indexing surfaces. A second piece overlies the backbone and includes mating surfaces engaging the indexing surfaces, aligning the second piece with the first piece. A receiving structure is opposite the mating surfaces for receiving accessory devices mounted thereon. Fasteners fixedly attach the second piece to the first piece to form a complete upper receiver.