Scorpion Evo 3 S1 Pistol Arm Brace Kit. - ENABLES THE ADDITION OF AN AFTERMARKET ARM BRACE OR OTHER PISTOL BUFFER 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 - THREADS ON ADAPTER (1-3/16x16) ARE STANDARD TO ACCEPT ALL AR-15 BUFFER TUBES/RECEIVER EXTENSIONS - IF USING A BUFFER TUBE OTHER THAN THE SUPPLIED KAK INDUSTRIES TUBE THERE ARE TWO THREADED ALIGNMENT HOLES (10x32 THREAD) FOR TUBES THAT USE A NOTCH FOR INDEXING. A 3/16 HALF DOG SOCKET SET SCREW WOULD BE USED FOR PROPER INDEXING. - GUNSMITH INSTALLATION IS RECOMMENDED