An upper receiver for a firearm, the upper receiver having: a hand guard section removably connected to the upper receiver via a locking mechanism pivotally mounted to the hand guard section for movement between a locking position and an unlocking position, wherein the locking mechanism requires only a single movement in one direction to move from the locking position to the unlocking position, wherein the hand guard section has peripheral device mounting rails located at a six o'clock position with respect to a centerline of the barrel and wherein the hand guard section is capable of being completely detached and removed from the upper receiver after the locking mechanism has been moved into the unlocking position; and a plurality of integrally formed rails located above the hand guard section and at three, nine and twelve o'clock positions with respect to the centerline of the barrel.