This complete upper receiver incudes an HBAR profile barrel that is laser marked "HBAR". Specifications for Aero Precision M4E1 16in 5.56 ATLAS R-ONE M-LOK Complete Upper Receiver with Flash Hider: Manufacturer: Aero Precision Caliber: 5.56x45mm NATO Overall Length: 26 in Barrel Length: 16 in Barrel Twist Rate: 1-7 Threads per Inch: 1/2x28 Muzzle Device: A2 Flash Hider Gun Model: AR-15 Condition: New Gas Block Journal Size-Diameter: .750 in Handguard Length: 15 in Gun Make: AR Platform Barrel Material: 4150 Chrome Moly Vanadium Bolt Carrier Group Included: No Charging Handle Included: No Weight: 48.5 oz Magazine Included: No Country of Origin: USA Features of Aero Precision M4E1 16in 5.56 ATLAS R-ONE M-LOK Complete Upper Receiver with Flash Hider Laser engraved T-marks Comes assembled, with port door and forward assist installed Accepts standard AR15/M16 components Handguard mounting platform is forged into the receiver Works with Aero Precision handguards but also fits most BAR-system handguards ATLAS S-ONE Handguard Features: Front and rear picatinny rail Eliminated center of top rail for weight reduction Indexing grooves in 12:00 position for positive grip control Compatible with mil-spec AR15 upper receivers and barrels MLOK attachment points Package Contents: Aero Precision M4E1 16in 5.56 ATLAS R-ONE M-LOK Complete Upper Receiver with Flash Hider M4E1 Enhanced Assembled Upper Receiver 15in ATLAS R-ONE M-LOK Handguard 16in 5.56 HBAR barrel Low Profile Gas Block and Mid-Length Gas Tube Product comes assembled This product is compatible with the following gun models: AR-15