This complete upper receiver incudes an HBAR profile barrel that is laser marked HBAR. - Includes: - M4E1 Assembled Upper Receiver - 15 ATLAS R-ONE M-LOK Handguard - 16 5.56 HBAR barrel - Low Profile Gas Block and Mid-Length Gas Tube - Product comes assembled - This complete upper does not include BCG or Charging Handle. - M4E1 Upper Features: - Forged from 7075 T6 aluminum - Precision machined to M4E1 specs - M4 feedramps - Matte black hard coat anodized Mil 8625 Type 3 Class 2 - 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 - Barrel nut and wrench are included - ATLAS R-ONE Handguard Features: - Full top picatinny rail for optional attachments - Quick disconnect sling socket at the 3 6 and 9 o’clock positions - 1.3 Inside diameter - 1.5 Outside diameter - Compatible with mil-spec AR15 upper receivers and barrels - MLOK attachment points - 16 5.56 HBAR Barrel - 5.56 Nato 16 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