In addition to the wise advise already provided, I would strongly recommend an "intensive" barrel cleaning process with something like VFG blue paste/felts. I have seen many barrels brought back from the dead following a good seeing too with this cleaning regime. I also know of several top shooters who have resorted to using the infamous copper brush coated with cleaning paste to achieve the same outcome, although I am personally not brave enough to be so aggressive with an airgun barrel.
Andy