Green CO2 bulb seal for Crosman 1911BB CO2 air pistol / airgun. For all versions of the 1911BB including the Tac1911bb and GI model.

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.