The SBUR04 receiver is a high quality, Mil-Spec upper receiver that is compatible with all of our lower receivers. It has a monolithic continuous top rail that is compatible with any AR-15 barrel nut and any AR-15 handguard out there, including those with rear facing tabs. The SBUR04 receiver is meant for .223, 5.56, .224 Valkyrie, .300 AAC Blackout and 6.5 Grendel Cal. The port opening is not large enough for .450 Bushmaster, .458 SOCOM or .50 Beowulf. The SBUR04 receiver is made from 7075-T6 aluminum, just like our lower receivers. It is anodized black and comes with a forward assist kit and dust cover installed.