The AR-15 lower build kit includes all the small parts needed to finish a lower receiver. It typically contains the trigger, hammer, disconnector, springs, pins, safety selector, bolt catch, magazine catch, buffer tube, buffer, buffer spring, and end plate. Builders drop these parts into a stripped lower receiver to complete the lower assembly. The kit fits mil-spec AR-15 lower receivers and works with standard upper receivers and magazines. Shooters choose one to simplify ordering by bundling the essential lower parts into a single purchase.