Constructed of an impact-resistant polymer and compatible with any standard AR-15 lower receiver, the CMMG® FourSix™ AR-15 4.6x30mm 40-Round Conversion Magazine safely feeds 4.6x30mm ammunition for reliable operation with your CMMG FourSix complete upper receiver (not included). The steel wire spring and polymer follower deliver smooth, jam-free operation, while the removable floor plate allows for quick maintenance and cleaning when required. Better yet, this magazine features a last-round bolt hold-open functionality for a clear indication of an empty magazine and faster reloads. Order today and enjoy reliable field performance at a low, affordable price.