Abstract: An assembly and method for cleaning the barrel of a pneumatic gun, such as a paintball marker. A supply magazine is provided that contains a plurality of loading ports. Each loading port is filled with a cleaning projectile. The supply magazine is attached to a pneumatic gun so that the supply magazine is interposed between the breach chamber of the pneumatic gun and its barrel. When a projectile is fired from the pneumatic gun, the projectile displaces a cleaning projectile from the supply magazine and advances the cleaning projectile through the barrel. The passage of the cleaning projectile and the accompanying paintball cleans any fouling from the barrel of the pneumatic gun.