SilencerCo Piston, M13.5 x 1 LH, Size XL AC626 SilencerCo pistons were designed to be easily changed for different host firearms or calibers. Pistons are built with 17-4 stainless steel" then heat-treated for maximum durability. The piston is part of the Nielson device that moves and allows semi-automatic cycling for unlocking pistol barrels. Throw a Fixed Barrel Spacer on for fixed barrel applications. SilencerCo pistons have female threading that attaches to our SilencerCo threaded barrels.