Sharps Bros SBUR04: The SBUR04 matches all of our lower receivers perfectly and has a mil-spec front end that is compatible with any AR15 barrel nut & any AR15 handguard out there, including those with rear facing tabs. This receiver is meant for .223, 5.56, .224 Valkyrie, .300 Blackout and 6.5 Grendel calibers. Port opening is not large enough for 450 Bushmaster, 458 Socom or 50 Beowulf. Made from 7075-T6 just like our lower receivers. Anodized black. Comes with forward assist kit & dust cover installed.