The UDP-9 pistol lower comes complete and ready to run with any 9mm AR upper. It accepts 9mm GLOCK magazines and features last round bolt hold open. Last round bolt hold open on an empty magazine (LRBHO) Accepts 9mm, .40 S&W and .357 SIG GLOCK(R) magazines Flared magwell for faster reloads Integrated, oversized trigger guard 7075-T6 billet aluminum lower with a matte black hard-coat anodized finish Works with any mil-spec upper receiver Threaded bolt catch screw For use with a 9mm bolt carrier assembly and a standard AR-15 hammer 5.4oz 9mm buffer kit B5 Systems pistol grip Patent pending Accepts all Glock(R) aftermarket magazines. We recommend using factory Glock(R) magazines. Please see this page for magazine compatibility