With paintball being the 2nd fastest growing growing extreme sport in the world (2006-2007), there are a lot of companies that would have you believe that their product is better, faster, more accurate, and will give you a huge edge over your opponents. This is mainly because they want to cash into the hard earned money that you get every paycheck. There are a lot of good companies out there that produce quality items, but they are few and far between. Here are some things to consider when you are looking for a barrel.

1. Barrel kits are as useful as the length of the insert and the smoothness of the control bore. Taking that into consideration, barrels with a bore than runs the length of the barrel will be consistantly more efficient than kits that have an insert that is 3" long.
2. Longer barrels will not make you shoot farther. A ball shot from an autococker at 300fps from a 21" barrel will travel the same distance as a ball shot from a spyder at 300fps with a 10" barrel. Well, the the ball from the autococker would travel 11 inches farther, but you see what I mean.
3. Rifling doesnt help. There are companies that offer rifling in their barrels, smiliar to that of a real gun. It's a nice idea, but the reality is that we are using liquid filled balls of gelatin. We are not using solid lead bullets. Side spin on a paintball will not make it more accurate at distances, it will cause horrible wobble at longer distances and doesnt increase accuracy at shorter distances.
4. The smoother the surface of the bore of the barrel, the better. High polish aluminum, ceramic, and stainless steel are your best bets...in that order of quality from lowest to highest.
5. Find a barrel that you can point or aim with easily. After all, if you can't aim well, you can't hit anything. Pointing tends to be easier with barrels that are longer because the eye will follow the length and tip of the barrel and improve hand/eye targetting. Shorter barrels are more challenging and take more time to adjust to, but can aim just as fast with practice.
6. Porting and muzzle breaks are used to disperse the air that builds up behind the ball. This allows the ball to regain its normal shape (the burst of air when shot distorts the shape of the ball). The porting and breaks also reduce the pop sound that occurs when the ball exits the barrel.
7. When considering barrel length, consider that longer barrels take more air to reach the same FPS as a shorter barrel. More porting also reduces the air efficiency of the barrel. 12" is the average. 14" - 16" may be used by back players that "warp" around bunkers.
8. In my experience (been playing since 1998) I have never had to use any other barrels than .685, .689, and .693. Barrel kits that offer 10 different sizes are unneccessary unless the kit is free or you are travelling the globe playing professional ball where temperatures and ball quality is going to vary dramatically. Save yourself the cash and buy just what you need.

Hope these little points are helpful in assisting you with choosing a barrel.