The JP Silent Captured Spring System is manufactured exclusively for Maxim Defense Industries to work with the Maxim Defense CQB Stock System. It requires an open channel in your bolt carrier group assembly for the guide rod to pass during fire.
Specifications:

Type	Capture Spring
Model	JP
Dimensions
Size
Material	Tungsten/Stainless Steel
Action
Trigger
Safety
Thread Pattern
Finish	Tungsten
Bolt Type
Hand
Hammer Type
Max Capacity