A modular integrated rail system for a firearm includes an upper hand guard having a dovetail rail extending longitudinally between the forward end and the rearward end thereof. The rearward end of the upper hand guard includes a clamp that removably secures the upper hand guard to the barrel nut of the firearm. The rearward end of the upper hand guard and the clamp include clamping surfaces configured to cooperatively engage the outer surface of the barrel nut. The upper hand guard further includes mounting channels that are used to mount accessories, such as a lower hand guard or a grenade launcher, to the upper hand guard. The rear end of the upper hand guard includes alignment tabs for automatically aligning the dovetail rail of the upper hand guard with the dovetail rail of the upper receiver during mounting. The tabs also prevent rotation of the rail system relative to the upper receiver during use.