Green CO2 bulb seal for the Crosman DPMS Panther Arms SBR M4 air rifle. This airgun takes 2 of these – one above each CO2 capsule.

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.