The SilencerCo AC59 Osprey/Octane Piston .5x28XL 9mm Stainless Steel is designed to be easily changed for different host firearms or calibers, featuring a 17-4 stainless steel construction that is heat-treated for maximum durability. As part of the Nielsen device, this piston moves to allow semi-automatic cycling for unlocking pistol barrels, with female threading that attaches to SilencerCo threaded barrels. When you need a longer interface, the XL piston provides additional length between the threaded barrel and the silencer, and for fixed barrel applications you can throw on a Fixed Barrel Spacer. This 9mm stainless steel piston is compatible with SilencerCo silencers such as the Osprey and Octane, ensuring reliable operation and smooth cycling across pistol platforms. The combination of material choice, precise threading 1/2-28 threads per inch, and the XL extension delivers versatility for users who swap hosts or calibers, maintaining performance without compromising safety or consistency. Whether upgrading a host pistol or configuring for a specific build, the AC59 piston offers durable, repeatable function with easy installation and compatibility with your SilencerCo threaded barrels.