but a longer barrel gives you a greater length of compression time i.e the time during which the expanding gas is acting upon the paintball and accelerating it. So there.
So, up to a point, longer barrel, longer acceleration time, greater muzzle velocity.
But on the other hand, if it's too long then you get interference from the gas and the ball can wobble, so the porting lets the gass escape and stops it affecting the flight of the ball adversely.
That's for firearms, with paintball markers you've a 300fps limit anyway, so it's just a matter of taste. And you have to admit the rifling looks cool.
After all theres only so much you do to a ball of gloop to make it fly straight.