The AR-15 lower receiver is the serialized component of the rifle that houses the fire control group, magazine well, and buffer system. It connects to the upper receiver via the front pivot pin and rear takedown pin. Builders install the trigger, hammer, safety selector, and other lower parts directly into the lower receiver. It accepts standard mil-spec upper receivers and works with buffer tubes, stocks, and pistol grips. Shooters choose stripped or complete versions depending on their build preference.