This 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.