Action Army VSR-10 Reinforced Piston. Fits Tokyo Marui VSR-10 sniper rifles, clone guns, JG BAR-10, etc.  This is the original style 45 degree piston, which is made for stock guns, triggers, etc. A great upgraded piston that will handle a lot.  

If you plan on using the Zero trigger, you will need the 90 degree piston instead of this one.