The Aero Precision Complete Upper Receiver epitomizes speed, efficiency, and dependability for fashioning a semi-custom AR15 rifle. Crafted with meticulous precision, this 5.56 NATO M4E1 Threaded Complete Upper boasts a 16 Mid-Length barrel and a 15 ATLAS S-ONE handguard. Meticulously machined, comprehensively assembled, and subject to rigorous test-firing by skilled Aero Precision armorers, it guarantees unparalleled performance.* Does not include Bolt Carrier Group or Charging Handle.Includes: M4E1 Threaded Assembled Upper Receiver - Anodized Kodiak Brown 15 M-LOK ATLAS S-ONE Handguard - Anodized Kodiak Brown 16 5.56 NATO Mid-Length Barrel Low-profile Gas Block and Gas Tube Standard A2 Flash HiderM4E1 Threaded Upper Receiver Features: New enhanced forging gives the upper receiver a "billet look." M4 Feed ramps .250 takedown pin holes Laser engraved T-marks Includes port door and forward assist installedATLAS S-ONE Handguard Features:Aero Precision's ATLAS attachment system is a secure and dependable mounting platform, maintaining a slim profile while still providing the strength and stability customers have grown to love from Aero Precision handguards. 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 1.3 Inside diameter 1.5 Outside diameter5.56 NATO Barrel Features: 5.56 NATO 1/7 twist, 4150 CrMoV, QPQ finish .750 gas block journal Mid-Length gas system M4 Feed Ramp Extension HP and MPI tested