This piston kit by Silencer Co. contains 5 pistons and three O-ring packs. The kit features .578x28 piston (1), 1/2"x28 piston (1), 16mm x 1 LH piston (1), 13.5mm x 1 LH piston (1), and 9/16"x24 piston (1). This kit is made to fit the Osprey and Octane suppressors by Silencer Co.