Weighing just 5.4 ounces, the R-15 magnesium Lightweight Lower Receiver is the lightest metal production AR15 on the market. For use with calibers like .223 Remington, 5.56 NATO, 300 Blackout, 450 Bushmaster, and many more. The R-15 is Mil Spec compatible and is designed with a tensioning screw, to ensure a secure fit with your upper, and a bolt catch screw in place of a roll pin. Protected with Tagnite and Cerakote, the R-15 Lower Receiver is the best choice for your ultra-light weight AR build.