This complete upper includes our ATLAS S-ONE Handguard! The S-ONE line of handguards are designed with size and weight in mind, providing the perfect addition to your lightweight build. Our proprietary ATLAS attachment system is a durable and dependable mounting platform, maintaining a slim profile while still providing the strength and stability customers have grown to love from Aero Precision handguards. Specifications for Aero Precision M4E1-E 13.9in 5.56x45mm NATO EM12.7 Complete Upper Receiver: Manufacturer: Aero Precision Cartridge: 5.56x45mm NATO Overall Length: 23 in Barrel Length: 13.9 in Barrel Profile: Government Gas System Length: Mid Length Barrel Twist Rate: 1-7 Threads per Inch: 1/2x28 Handguard Type: A2 Muzzle Device: A2 Flash Hider Gas Block Journal Size-Diameter: .750 in Barrel Material: 4150 CMV Steel Bolt Carrier Group Included: No Charging Handle Included: No Magazine Included: No Country of Origin: USA Features of Aero Precision M4E1-E 13.9in 5.56x45mm NATO EM12.7 Complete Upper Receiver M4E1 Threaded Receiver: New enhanced forging that gives the upper receiver a billet look New picatinny profile that blends seamlessly with our Enhanced Series Handguards M4 Feedramps .250 takedown pin holes Laser engraved T-marks Includes port door and forward assist installed Atlas S-One Handguard: Front and rear picatinny rail Eliminated center of top rail for weight reduction Indexing grooves in 12:00 position for positive grip control Quick disconnect sling socket at the 3, 6 and 9 o'clock positions Compatible with low profile gas blocks 1.3in Inside diameter 1.5in Outside diameter Compatible with mil-spec AR15 upper receivers and barrels 13.9in 5.56 M4 CMV Barrel, Mid-Length: 5.56 Nato 13.9 Barrel, 1/7 twist, 4150 CrMoV, QPQ, gas port drilled .750 gas block journal Mid-Length Gas System M4 Feed Ramp Extension HP and MPI tested Standard A2 flash hider Package Contents: Aero Precision M4E1-E 13.9in 5.56x45mm NATO EM12.7 Complete Upper Receiver M4E1 Threaded Assembled Upper Receiver 12.7in ATLAS S-ONE Handguard Of Choice 13.9in 5.56 Mid-Length CMV Barrel Low Profile Gas Block and Pistol Length Gas Tube Product Comes Assembled