As projectile launchers are used, the projectiles (balls) may transfer some dirt to the inner surface of the barrel. The design of the launchers depend on a close fit between the ball and the barrel, so any additional material will cause friction and launch irregularities.
To clean the barrel, try two things:
1) Wipe the inside of the barrel with a clean rag and the commercial solvent “Goo Gone” or “Goof Off”.
Alcohol will not remove wax, and so is not recommended.
2) If the solvent does not remove enough material, wipe the inside of the barrel near the lip with 1500 grit sandpaper.
It is important to use very fine sand paper; if a coarser grit is used, the inner dimension of the barrel may change too much.
Occasional cleaning of the projectiles with solvent (as noted above) will help keep gunk out of the barrel.
Do not use oils to clean the projectiles as the oil can damage the launcher.