Paintballs don't deform when shot! One of the big pball companies (forget who and I can't be arsed to find the link) hired the Kodak high speed camera lab - the sort of place that produces those really cool films of real bullets being shot from real guns - and filmed a paintball being fired. Result? They don't deform.
Rifling (ported or using internal ridges) doesn't work, that's why 'rifled' barrels don't get made anymore. IIRC, you'd have to have a barrel about 14 feet long to spin the ball up to a viable speed.
A longer barrel isn't more accurate, just quieter, it doesn't make you more accurate or shoot further other than being a couple of inches closer - it's basic ballistics, if your paintball exits the barrel with the same muzzle velocity then it will follow the same trajectory whether you're using a 14" or a 140" barrel! A paintball is only accelerated in the first ~6-8 inches of a barrel, anything after that is just to provide porting to reduce the acoustic signature of the shot and to enable you to win the "who's got the biggest" competition.
