Tape or gun graffiti is good but the best i've seen was a member of nexus when they were shooting egos in 06 tapped a dye Ultralite plastic tube over the tip of the barrel to keep rain out and still let it port.
The reason to cover the porting is because the water will enter the barrel causing a restriction and extra friction unevenly on the surface, the can cause swelling and at the very least will cause the ball to reduce in speed and gain abit of spin in a random direction.
Remeber kids, always wrap your barrel if it is getting wet.