The Airsoft GI Extreme VSR-10 Hop Up Bucking is a great upgrade to ensure a better air seal and proper backspin on the BB during flight. The material used on this hop bucking is called "HNBR".
Here is a short description on HNBR, Hydrogenated nitrile rubber (HNBR) is a viable alternative to polychloroprene for many automotive applications such as timing belts, seals and hoses. It has excellent mechanical, thermo-oxidative and chemical resistant properties and an excellent operating temperature range.
Note: Does not include spring, spring guide, cylinder kit, sear set, piston, barrel, or threaded barrel adapter.