There is an o-ring inside the barrel. This provides the airtight seal between the probe (part that pushes the bullets into the barrel) and the barrel.
This ring is essential for the proper functioning of the rifle.
How can this o-ring break or pop?
Usually this is due to lack of maintenance or simply the old age of the o-ring.
It is essential with compressed air rifles to periodically grease the raceway with silicone grease such as (for example) Hill NLGI2 or Abbey Silicone Gun Grease.
This can easily be done by greasing the probe and moving it back and forth along the raceway.
This really does not have to be done often and how often differs per rifle, but it is essential for long-term preservation.