The lower receiver is the serialized heart of an AR build — everything else is parts. Stripped lowers from Aero Precision, Sharps Bros, and San Tan Tactical give builders a blank canvas, while complete lowers from BCM and LWRC arrive with trigger, stock, and grip installed. Most are machined from 7075-T6 aluminum in AR-15, AR-10, and pistol-caliber patterns, including 9mm lowers that accept Glock magazines. All transfers follow the standard FFL process.