Aero Precision APAR620201AC: The EPC-9 upper receiver is the perfect addition to any AR9 build. Including a last-round bolt hold-open mechanism and compatible with most standard AR15 barrel nut and handguard combinations, this upper receiver will help take your pistol caliber to build to the next level. Features last round bolt hold-open mechanism Accepts most AR-15 compatible barrel nuts and handguards. Additionally, the upper has a dimple above threads to allow proper handguard timing and uses a standard AR15 takedown/pivot pin lug pattern. Machined from custom forged 7075-T6 aluminum Laser engraved T-marks.