The AR-15 lower part kit contains the small components needed to finish a stripped lower receiver. It includes the trigger, hammer, disconnector, springs, pins, safety selector, magazine catch, and bolt catch. Most kits also include takedown pins and a pistol grip. Builders install these parts into a mil-spec lower receiver to complete the lower assembly. The kit works with standard AR-15 upper receivers, buffer tubes, and stocks. Shooters use one to source all the essential lower hardware in a single purchase.