Primary Weapons Systems, Piston, Fits 13.5x1LH, Anodized Finish, Black NEW WARNING Warning WARNING CA PROP 65 WARNING: This product may contain chemicals known to the state of CA to cause cancer, birth defects or other reproductive harm. 13.5x1 LH Anodized Finish Black Product Description Primary Weapons Systems was formed by firearm enthusiasts in 2008. Starting out by producing muzzle devices, we tested our compensators on all different types of rifles - from M4s to AK-47s. After using both styles of rifles in testing with high round counts, the difference in maintenance needed was both astounding. From that basic realization, the PWS long-stroke piston system was born; keeping the core values of the AR-15 design, while also lessening the need for maintenance in high-volume fire situations and adverse conditions. The 13.5x1 LH piston allows the use of PWS suppressors on 13.5x1 LH threads.