The AR-15/M16 M4 Receiver Extension Castle Nut is an essential component for securing the buffer tube assembly on AR-15 platform rifles.AR-15/M16 M4 Receiver Extension Castle NutCompatible with AR-15 M16 and M4 rifle platformsSecures receiver extension/buffer tube to lower receiverStandard threading for proper fitmentDurable construction for reliable performanceThis castle nut is designed for proper assembly and function of the buffer tube system ensuring secure attachment of the stock assembly to the rifle's lower receiver.