SRS Variable Mass Piston – NBR piston cup (70 Deg) – CNC AL 6061 T6 piston head – CNC AL 6061 T6 piston body – CNC steel piston end – CNC aluminium flyweight (4 pieces) – CNC steel flyweight (4 pieces) – PTFE grease – Tools For Pull Bolt Only This special piston will allow you to tweak the muzzle energy of your SRS by adjusting the piston mass from 40 to 137 grams with 15 different configurations.