The Luth-AR AR-9 9mm Carbine Buffer Tube Assembly is a complete receiver-extension kit purpose-built for 9mm pistol-caliber AR-9 carbines. The 9mm-weighted buffer is matched to the slower bolt-velocity of blowback PCC systems, which keeps cycling smooth and reduces wear vs. dropping a 5.56-spec buffer into a 9mm build. Fit: AR-9 9mm carbine builds with mil-spec lower receivers and mil-spec diameter carbine stocks (stock not included). Includes: Mil-spec diameter carbine receiver-extension tube - hard-coat anodized aluminum 9mm-specific carbine recoil buffer Carbine-length recoil spring Steel receiver end plate Steel castle nut Luth-AR is a US manufacturer. Castle nut should be torqued and staked. Verify your lower is mil-spec before install; commercial-spec tubes and stocks are not interchangeable with this assembly.