The CMMG Lower Parts AR15 Gun Builder's Kit includes most of the necessary mil-spec parts to build up a lower receiver group. The kit includes: bolt catch, bolt catch plunger, bolt catch roll pin, bolt catch spring, buffer retainer, buffer retainer spring, hammer pin, magazine catch, magazine catch button, magazine catch spring, pivot pin, pivot pin detent, pivot pin detent spring, selector, selector detent, selector detent spring, takedown pin, takedown pin detent, takedown pin detent spring, and trigger pin. The kit does not include a pistol grip or trigger guard. The CMMG Lower Parts AR15 Gun Builder's Kit is an economical way to get all the small parts you need by not having to buy them individually.? Manufacturer model #: 55CA601. Includes most parts to build a lower receiver group Mil-spec dimensions Economical