While you can use a standard buffer retainer pin and spring with your Silent Captured Spring, installation and removal is difficult to say the least. Until now, that's left the only option of storing the SCS loose. Fortunately, we have a solution. Compatibility: Like a standard buffer retainer pin, this design is not compatible with Variable Mass Operating System (VMOS(TM)) carriers. This new buffer retainer pin design allows for easy installation and removal of the Silent Captured Spring while ensuring unerring retention. Your SCS will never drop unexpectedly out of your lower again. Plus, this design works equally well with a standard AR buffer and spring. I Mfg: J P Enterprises