The probe is attached to the bolt and pushes the pellets out of the magazine into the barrel.
This o-ring sits around this probe and ensures an airtight seal in the barrel when shooting.
It can wear out over the years, so a spare is handy!
If this ring is missing, your pellets will not come out of the barrel and you have a chance of double loading.
Suitable for the 5.5mm variants of the Gamo HPA, Coyote, Urban, GX40 and the BSA Buccaneer.
This ring is also part B (16-6560) on the Gamo Coyote Exploded view.