This kit contains all the necessary parts to mount your Mil-Spec dimension stock assembly to your lower receiver. Receiver Extension - Mil-Spec, 6 position (machined from 7075T6) Receiver End Plate Lock Nut (Castle) M4 Carbine Action Spring Carbine Buffer Assembly This kit is compatible with the plastic stocks from; BCM®, VLTOR, LMT, Magpul® CTR-M, Stag Arms, Colt