Green CO2 bulb seal for the Cybergun CO2 Air Pistols.

If your cybergun CO2 air pistol has a green seal where the CO2 butts up this is it.

This is the seal against which the CO2 capsule fits and the most common to be replaced.
Generally needs to be replaced if a CO2 capsule has been left in the airgun for a long time, which causes leaks when the CO2 is replaced.