These Pistons are required for use with the Revolution 9 and 45 silencers for pistol function. The Revolution 9 ships with a 1/2x28 piston and the Revolution 45 ships with a .578x28 piston. It is important to intermittently clean and lubricate the exterior of the piston to ensure smooth reciprocation with the O ring in the piston retainer. O-Rings should be replaced intermittently when worn to ensure alignment of the silencer with the booster assembly. Failure to replace damaged or worn O-rings could result in a baffle strike or damage to the piston components.