The mil-spec buffer tube kit includes all of the necessary parts to mount your mil-spec dimension stock assembly to your lower receiver including: Mil-Spec 6-position receiver tube, precision machined from 6061/t6 aircraft grade aluminum, fits most .223/5.56 receivers, 3oz mil-spec buffer, mil-spec buffer spring, steel castle nut with 4 slots for castle wrench and hole for older style wrench, steel end plate.