One of these is the starting point to design your customized Glock Carbine Conversion Unit (CCU). Each base model (shown below) comes with a 6″ Standard Rail and Molded Fore Grip. If you have your own M4 stock, buffer tube, castle nut and lock ring, our M4 Adapter will accept both commercial and Mil-Spec tubes. Every unit is test fired before it ships. Other rail choices are available which may replace the Standard Rail/Molded Fore Grip. All of our rails are Picatinny. Choose from our ACCESSORIES page to customize your CCU.