OK. So aerodynamics for paintball has a few variables:
Ball shape + fill
Breech and barrel match to the range of ball.
Porting (to some degree)
Consistency of application of propellant (how your marker powers the ball out of the breech)
For the vast majority of us, ball shape is the most important variable. It is the one that changes the most and the one that we have the least control over. Everyone agrees that non-round out of shape and underfilled balls are the main cause of inaccuracy.
Matching a few paintballs to barrel backs in decreasing size until they just fit through is the normal method for selecting an appropriate barrel for the paint you want to shoot. This barrel matching method was proven to be the least accurate method by a bloke called cockerpunk (his channel is https://www.youtube.com/user/gerglmuff if you are interested). In the remainder of that test, underboring by some margin was proven as the most accurate method, with overboring not terribly far behind.
Since underboring with fragile paint increases the chance of breaks during the ball's travel IN THE BARREL (n.b. not in the breech, where no amount of barrel matching will save you), overboring is the next best thing for guns shooting high rates of fire. The game between barrel manufacturers and paint manufacturers wobbles around a bit but generally most people's barrels are either .689 or .693 and paint manufacturers try to make the best match considering the environment their product is going to be used in.
Drilling holes in your barrel to allow air to escape behind the ball is called porting. Some barrels have fancy patterns and increasing/decreasing numeracy of ports along the barrel and most designs get a decent accuracy. The theory behind porting is that once the ball exits the barrel, the air behind it escapes and nudges the ball off course. I haven't read anything that disproves this theory but I just don't believe it. Many old pump guns had no porting and some of them shot straighter than most modern markers. In some ways the variation of porting and the negligible effect on the accuracy of paintballs shows that once the ball gets to the ported section of the barrel, nothing much changes with it's flight and two inches more barrel length won't increase your accuracy anyway.
The consistency of air application is a variable I don't really know about. This is the controllable variable that affects vertical accuracy, although vertical accuracy is governed mainly by paint match and consistency of shell. I'm still waiting for someone to do a trajectory plot of spool valve markers vs poppets vs whatever. I think there's a lot of myths to be busted in this area.
My opinion is that there are so many other way more important factors than the length of your ported section of your barrel so just pick the one that suits your style of playing or the one you think looks coolest and overbore where you can.
Soz that is a major schpeel and i'm not going to read through it to check for terrible English.
