The SBUR04 matches all of the Sharps Bros 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 BLK and 6.5 Grendel calibers. Port opening is not large enough for 450 Bushmaster, 458 SOCOM or 50 Beowulf. Made from 7075-T6. Mil-Spec Type III hard coat anodize. 'SHARPS BROS.' discretely engraved on both sides. Comes with forward assist kit & dust cover installed. Weight: 9 ounces. Mfg: Sharps Bros