This Article is written by Glenn Sweeten but all of the great info is being used with permission of Ronald Sonedecker a.k.a. Miscreant, owner of X-odus Paintball.

        I have had many people ask "what is the BEST barrel?", or "what is the BEST paint?" the answer...none.  The best balls and best barrels depend on many things: weather, humidity, heat, condition, materials, weight, and most importantly, bore size.
        Ok, so what is bore size?  In barrels, bore size is the diameter of the inside of the barrel.  In other words, the size of the hole.  In balls, the bore size in more often and more correcly reffered to as ball id.  This is the measure of the outside of the ball.  To get the best results the bore size of the barrel, and the id of the ball should be matched up as close as possible.  This forces the air to push the whole ball instead of parts of it, and does not let air escape out around the ball.
        Now, that is all well and good right?  Wrong.  The problem is paintballs are made of liquid.  Liquid is inconstant, this means the bore size changes.  Balls may become bigger when it is hot out, or when there is alot of humidity.  Balls shrink in size when it is cold out of or when there is little humidity.  What does this mean?  It pretty much means get a couple barrels of different bore size for different balls and conditions.  Balls also vary in bore size depending on their brand.  When you are about to buy a new barrel, pick one that is about the same bore size as the balls you shoot the most.  Also get one that feels right, is the weight you want, and is the length you want.  Here is a list of different bore sizes:
 

Glenn Sweeten
MPN Columinst