VMOS Bolt Carrier Features The Variable Mass Operating System (VMOS(TM)) 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. This standard version of the large-frame VMOS(TM) carrier includes three steel weights two of which can be exchanged for heavier tungsten weights. Requires DPMS-pattern bolt firing pin and cam pin. VMOS Bolt Carrier Specifications Brand: JP Rifles Model: VMOS Bolt Carrier Frame: Large (.308) Finish: QPQ Black Material: Stainless Steel Version: Standard Weight: 14.40 oz