Green CO2 bulb seal the UX XCP air pistol by Umarex. Note: this part is not suitable for the blowback version of this pistol!

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.

The most common cause of leaks on these pistols.