Upgrade your Paintball Barrel for improved accuracy

You should be able to remove the barrel from the gun to allow upgrading.  Stock barrels usually aren't the best.  Aftermarket barrels offer a better paint to barrel match, are quieter due to porting, have better honing (the finish of the bore is smoother resulting in less friction), or look better.

Paintball to barrel match
Put a ball in the barrel and blow it out.  If it rolls out (too small) or gets stuck (too big), you have a bad match.

Paintballs expand when warm or humid, and contract when cold or dry.  You may want to have three barrels for smaller, normal, and larger paint.  That way you can match the paint to the bore while on the field. Often, the field will require you to buy their paint; this supports the field, and everyone is using the same paint. The Empire Ceramic, and the Freak barrel systems gives you a barrel with different diameters to match the paint you are using, but this system may cost as much or more than the gun itself if you buy a cheap paintball gun.

Barrel length
You will do fine with a barrel between 10" and 14".  Shorter is less accurate and louder; longer has more friction and uses more gas to get to speed, is clumsy in the field, harder to snap shoot, and easier to see popping out from behind a bunker.

Low pressure (less than about 200 PSI) guns may require a longer 12-14" barrel to reach speed.  High pressure guns (about 800 PSI) may only need 10-12".  If you use low pressure and a really short 6-8" barrel, no porting is mandatory or you won't get up to speed, or use too much gas. Less speed means less range, and count on less accuracy.  Longer barrels do not increase accuracy, but they are quieter.

Many players report 14" is the best compromise because it is long enough to poke through brush and is still easy to aim from the barrel. 16" is starting to get unwieldy and will add drag, slowing the ball down and making the gun less efficient, and may add unnecessary spin which will hinder accuracy.

Porting
Holes placed in the barrel make the gun quieter, and reduce turbulence making it more accurate. Once the paintball reaches the ports, the acceleration phase is over. A shorter acceleration length will require more gas for the paintball to come up to speed. This is because the ball has less time to come up to speed, so the gas has to push harder in less time.

Honing
Honing, or the finishing/polishing process of the barrel, determines the amount of friction.  More friction means the ball moves slower, and that may lead to more paintball breaks in the barrel. A ceramic barrel such as the J&J Ceramic is made to reduce friction.

Quietness
No barrel is really quiet. In the reviews, some barrels are quieter than others; it is all relative.

Self Cleaning Versus Easy to Clean
When you break a ball, the paint in the barrel will cause inaccurate shooting. A self cleaning barrel is one that will recover its accuracy after a number of shots, usually between 5 and 10. Sometimes you will have to turn the gun over to dry-fire the gun to remove the paint from the port holes of the barrel. This may require plugging the end of the barrel with your hand (but be sure to shoot once to be sure there are no paintballs left in the chamber). Barrels that do not self-clean will require using a squeegee during the game, but a squeegee doesn't remove paint from the port holes, and you may have to wait till game break to wash the barrel with water.

An easy to clean barrel is one in which you can easily remove the paint from the barrel and port holes (if any) after game play.