Brownells AR-15 Mil-spec receiver 6 position extension tube has the correct 1.148" outside diameter to ensure proper fit of adjustable mil-spec carbine stock. Allows you to install a mil-spec stock on a commercial carbine, convert a rifle with a fixed A2 style stock to an adjustable M4 stock, or complete a custom build. Six machined notches on the underside prevent the stock locking pin from slipping, yet still offer outstanding ease and flexibility in setting the buttstock length. Precision machined from strong, lightweight 7075 aluminum, then hardcoat anodized with a hard, wear-resistant matte black finish.