An barrel (or barrel section) for a pneumatic gun can comprise a plurality of grooves and ridges arranged in parallel straight lines. The ridges may have a substantially arch-shaped cross-section with a rounded upper surface to provide minimal contact with a projectile such as a paintball being launched through the barrel. The grooves preferably provide channels to permit water and debris to be expelled from the barrel without interfering with the projectile travel. A plurality of apertures may be formed through a sidewall of the barrel along one or more of the grooves to further help expel water and debris, to impart spin to the projectile, and/or to improve a sound signature of the barrel.