The KAK Industry AR-15 Carbine Buffer Tube Kit gives you the complete mil-spec receiver extension assembly for a collapsible-stock AR-15 build. Everything that goes between the lower receiver and the stock is in the box. The 6-position buffer tube is machined from 7075 aluminum and finished in black military hard coat anodizing. It is mil-spec diameter, so it accepts mil-spec carbine stocks, and it takes standard AR-15 buffers and springs. The kit includes a carbine buffer and a chrome silicon action spring, so the recoil system is matched out of the box. The castle nut and end plate lock the tube to the lower receiver and retain the takedown pin detent and spring. Stake the castle nut into the end plate once it is torqued to keep it from backing off. Kit includes: 6-position mil-spec carbine buffer tube Carbine buffer Chrome silicon buffer spring Castle nut End plate Specs: 7075 aluminum tube Black military hard coat anodized finish Accepts AR-15 buffers and springs Mil-spec and commercial tubes are different diameters. Confirm your stock is a mil-spec fit before ordering, and function-check your rifle after installation.