Silent Captured Spring (patented) 9mm AR Version The 9mm Silent Captured Spring provides the same improvements of smoother operation and reduced cycling noise as our gas gun models. Compatibility The 9mm SCS requires an open channel in your 9mm bolt assembly for the guide rod to pass during fire. For most bolts, this will require you to remove the pinned-in central mass. To function with the SCS, this central mass must be removable, and the bolt must still be at its full length without it. Refer to the instructions link below for more.