TAC-41 Variable Mass Piston - Please note you can use the SRS Variable mass piston, all you need to do is swap the rear piston sear from the Stock TAC-41 Piston to the Variable mass piston. 
 
Whats included:
4 x Red Aluminium Weights (each weighing 8.2g)
4 x Grey Steel Weights (each weighing 24.2g)
Piston with new Silverback quieter piston head
2 x Tightening rods
Tightening nut
Silverback grease
 
This kit will enable you to customise your piston weight from the standard piston (being 39.8g up to 136.6g with all 4 Grey weights attached)