The hop packing is an intermediary component that preserves the air seal between the inner barrel and air seal nozzle. It is also the material that contacts the top of the BB to provide backspin. It is installed over the inner barrel and placed inside the hop-up chamber. The rubber comes in different degrees of hardness to cope with the different weights of BBs.
The hop packing is very important to carry the compression over into the inner barrel. Poor air seal against the hop packing can ensure massive losses in muzzle velocity and inconsiatent readings.