FMK Firearms AR1 Extreme AR-15 Poly Lower Features: The FMK Firearms AR1 lower receiver is a multi-caliber, AR15 Lower Receiver, manufactured in the USA of a high impact proprietary composite polymer. This state of the art polymer provides superior strength, unmatched durability and is extremely lightweight at only 4.3oz compared to competing polymer lowers. The AR1 eXtreme is by far the strongest composite polymer lower on the market today. The key to any AR build is the foundation and on an AR15, that is the lower receiver.