Silicon chrome spring made up of variable pitch. Both PDI made piston (inner diameter 13mm) and PDI spring guide (shaft outer diameter 9mm) are required for use. Marui VSR-10 original piston and guide are both incompatible. Although the spring designation refers to the FPS when using PDI Precision Cylinder Hard Set and 0.2g BB bullet, the FPS may vary according to the setting of each individual gun and so it's only for reference only and please exercise caution.