SilencerCo pistons are designed for easy swapping between different host firearms or calibers. Made from 17-4 stainless steel and heat-treated for increased durability, these pistons are part of the Nielson device that moves to enable semi-automatic cycling and unlock pistol barrels. For fixed barrel applications, simply add a Fixed Barrel Spacer. SilencerCo pistons feature female threading that connects to SilencerCo threaded barrels. RSR -