Crosman Airgun Forum: "If your 1377 doesn't shoot accurately, its not because the barrel is 'dirty'. Earlier posts discussed lapping, bore paste, etc, all things that should not be done to the very fine rifling in a 1377. If anything, the muzzle end of the barrel needs to be recrowned, and the breech end checked for burrs. This is very common in the Crosman guns, and almost all need at least some barrel work to be really accurate.

As for when you should clean your airgun, well, maybe never. I have guns found in original condition that are going on 70 years old, they've never been cleaned and shoot better than any new gun. I have new guns that have thousands of pellets through them, never been cleaned, and accuracy is better than when new. The only reason to clean an airgun bore is if its a springer thats been dieseling heavily. That causes a carbon and oil sludge buildup in the barrel that needs to be removed. "