The KAK 9mm Heavy Bolt Carrier Extension for the Law Tactical AR-15 Folding Stock Adapter is for special subsonic applications for 9mm and 45 ACP builds where the internal BCG weight must be removed. The necessary retaining O-Rings and Bumper O-Rings are installed, making this a direct replacement for the factory model. These extensions can be swapped quickly and easily back and forth. Note: Not designed to be fired when folded- damage to the weapon and user will occur