Enables the addition of an aftermarket arm brace or other pistolbuffer tube accessory. Adapter is made of 6061 T6 aluminum, flat black anodized. Buffer tube made by KAK Industries specifically for fitment of an aftermarket arm brace. Threadon adapter(1-3/16x16) are standard to accept allAR-15 buffer tubes/ receiver extensions. If using a buffer tube other than the suppliedKAK Industries tubethere are two threaded alignment holes(10x32 Thread) for tubes that use a notch for indexing. A 3/16 half dog sockett set screw would be used for proper idexing. Gunsmith installation is recommended.