E3 Firearms COMPLETE 5.56 UPPER (Mil-Spec) 7075 Forged Aluminum A-3 Flat Top with T-Marks FULL AUTO Bolt Carrier Group 8620 steel Bolt is 158 Carpenter Steel Bolt is HPT, MPI Front Rail Gas Block Barrel: H-bar Barrel Melonite Finish 1:7 Twist 5.56 Chamber (also accepts .223 Mil Spec M4 Feed Ramp HPT & MPI Carbine Length Gas, direct impingement system Gas port drilled and tuned for optical cycling Our 16″ H-bar Barrels which are more rigid will eliminate any vibration and improve accuracy while at the same time offer lighter weight of that approaching an M4 as opposed to the heavier bull barrels. Our barrels have been professionally developed for accuracy and competition shooting. With a Melonite finish applied to the steel through salt bath ferritic nitrocurizing our barrels not only add to the life of the barrel by over twice of that of a chrome lined barrel but also improves the accuracy, decreases the friction & heat increasing velocity. All of our uppers are available with the standard M4 Melonite barrel, the Stainless Steel barrel or our Pencil Barrel. We try to keep all varieties in stock. If the one of the barrel choices is not in stock we will build your upper receiver with one of the barrels in stock and notify you. If you require a specific barrel, there may be a delay in building your upper receiver.