its a hotly debated topic, do a search in google. but as i understand it, most markers need i beleive its 8" or so to accelerate and a further 2" to become accurate after that is mainly porting to reduce noise but the further the paint has to move down the barrel beyond the initial 8" length of acceleration the more friction occurs which results in a drop in efficiency. so the ideal barrel length is about 12" with 2" of porting, or 10" without. next is the material debate. its argued that of all the popular metals used in paintball brass offers the least friction to the balls, followed by stainless steel then aluminuim. Dont always assume the most expensive is the best either, if you can try them out or checkout
www.pbreview.com . i would like to add that barrel honing has come on alot and a good quality aluminium barrel will not be significantly less efficient. also theres a debate about the ideal shape of a barrel, and a paintball guru i have a lot of faith in Glenn Palmer suggest an eliptical shape is best and a good quality barrel is far more important than an exact paint/barrel match which is all the rage these days, sceptics might suggest this is a marketing ploy, not that we ballers are suseptable to those!
so to sumarise, no longer barrels arnt more accurate, the only thing they are more, assuming all other things are equal and the exess length is ported, is quite.
to be honest most of what you hear is down to marketing, just learn about the topic and for you own opinion.
id also like to add, long barrels may infact reduce accuracy, long story short, paintballs move very slowly compared to other missile fired down barrels and the longer it takes for the projectile to get out of the barrel the longer your movments and the movments of the object firing the projectile have to effect where the barrel is pointing, sounds mad but its true. also long barrels can unbalance the marker and also become cumbersome when firing in confined spaces, as in behind a bunker, etc.
also just do a little research into barrel honing, and try to find out who uses what techniques.
further reading
http://paintball.about.com/od/paintballbarrels/a/barrelmaterial.htm
http://www.palmer-pursuit.com/techpages/barrels.htm
http://www.pbreview.com/products/cat/30/
here's an interesting extract.. i took the names out so i dont get in trouble...
using military gun-barrel honing technology. Only (deleted) and (deleted) have a honing process this high quality. Unfortunately, (deleted) barrels have too much porting and don't have adjustable bore. Gun honing involves a long system of heating and tempering until the barrel comes out with a mirror surface and perfect caliber.
(deleted) uses a cheap technology called "extruding" to hone their barrels. First, a block of aluminum is dragged over a .68 caliber cylinder of metal to produce a nice cirle with a rough surface. They polish them to a simulated mirror finish, but a bore tester proves that (deleted) barrels don't have what it takes to be as accurate as (deleted)
