For difficult setups like suppressed rifles and exotic caliber chamberings, some degree of mass regulation is required for optimal performance. This is especially true in the absence of an adjustable gas block. Residual gas and excessive fouling can grind your rifle to a stop. One size never fits all when it comes to gas-operated rifles. The Variable Mass Operating System (VMOS) carrier ensures reliability through additional reciprocating mass but provides more custom tunability. Combined with the Silent Captured Spring, multiple masses are exchangeable to perfectly tune your operating system weight. Package includes carrier, enhanced gas ring and instruction sheet. Notes:VMOS carrier use the same steel and tungsten weights as Gen 2 Silent Captured Spring units. Only the forward mass is reconfigurable. The rear steel mass cannot be exchanged. This carrier will not cycle with a standard buffer retainer plunger installed. Standard buffer/spring components must be installed "loo...