Please vote for this guide to let me know whether or not it was helpful to you.  Thanks!

So, you've decided that your stock barrel just isn't good enough and you want to be more accurate?  There's a few things you have to understand about Paintball guns, paintballs and barrels to help you decide which one is right for you and your style of play.  Most manufacturers have their own set of personalized "threads" for their specific brand of gun.  The threading is the section of barrel that attaches or screws into your gun.  Some share the same, but most do not.  The following is a list of the most common guns and their threading.  If your gun is not listed, do some more research (ie look on the manufacturers website or in the owner's manual): 

AUTOCOCKER THREADS:

ALL Matrixes including DM's, PM's
Freestyles
Egos
Eteks
Intimidators
newer bushmasters
Dark shockers
Evil shockers
Shocktech shockers
Autocockers
Protos
ONYX
Fusions
Legends
ULE mags
Minions
quests
cyborgs
aliens
AKA guns
nme

SPYDER THREADS:

ALL kingman markers
most blowback markers
ACI markers
Ariakon
Armotech

Spyder clones (Icon, etc)
Brass eagle
Viewloader
diablo
dragun
JT
Odyssey
pmi

ANGEL THREADS:

ALL wdp markers PRE 06 are old style
ALL wdp markers made in 06 are new style thread
A1 offers spacer to fit "new style"threads

IMPULSE THREADS:

Impulse
RL shockers
ION
Nerve
PIMP

SHOCKER THREADS:

OLD school shockers
03 based shockers
MOST pl shockers

BUSHMASTER THREADS:

pre 04 ICD guns

TIPPMANN A5 THREADS:

Procarbine
A-5

TIPPMANN 98 THREADS:

98/98c/98 PRO

Now that you have figured out which threads you need, let's look at the length.  Anything over 16" will cause you to use more air/CO2 to propel the paintball and will cause a drop in efficency.  This is relatively counter productive.  Contrary to popular (uninformed) belief, the longer the barrel is will not improve your accuracy.  You should really try to keep your barrel length 16" and under.

In order to truely improve your accuracy, you need to establish a good "paint to bore" match.  Each paintball is not exactly .68 caliber.  Each paintball and each manufacturerhas sizing differences and flaws that change the size.  Paintballs truely vary in size from .679 - .694.  When the paintball does not sit fully in the barrel it allows excess air to escape around it affecting the flight.  Here is a complete list of common brands and their sizes:

32 Degrees Team Color .689 - .691
32 Degrees Team Color(Winter Fill) .689 - .691
Adrenalin .688 - .691
Brass Eagle Afterburner .692 and
Brass Eagle Blue Streak .688 - .690
Brass Eagle Paintballs .689 - .691
Brass Eagle Top Brass .689 - .691
Cobra Snake Eggs .688 - .691
Core Action .689 - .691
Core Ice .688 - .691
Core Paintballs .689 - .691
Core Premium .688 - .691
DBX Practice Grade .689 - .691
Diablo Blaze .687 - .690
Diablo Crucifire .689
Diablo Dark Leigon Night .687 - .689
Diablo Dusk .687 - .691
Diablo Heat .693
Diablo HellFire .682 - .685
Diablo Inferno .688 - .691
Diablo Midnight .688 - .690
Diablo Nightmare .687 - .689
Diablo Recsport .687 - .690
Diablo Splash .687 - .690
Direct Hit .687 - .690
Drakula Attack .687 - .690
DraXxus Arctic Inferno .687 - .690
DraXxus Blaze .687 - .690
DraXxus Blitz .687 - .690
DraXxus Competitive Edge .687 - .690
DraXxus Dusk .688
DraXxus Hellfire .687 - .690
DraXxus Inferno .688 - .689
DraXxus Midnight .687 - .690
DraXxus - Recsport .687 - .690
DraXxus Sunflare .687 - .690
DraXxus - XBall .687 - .690
Dynasty Dragon .689
Dynasty Dynamic .689
Eliminator .689
Empire Fuel .687 - .689
Fuego Premium .689 - .691
Game Face Scorch .688 - .689
Great American .688 - .690
Great American - Banana Ball .688 - .690
Great American - Sour Apple .688 - .690
I & I Sports Premium .689 - .691
JT Carnivore .689 - .691
JT Elite .689 - .690
JT Freedom .689 - .690
JT J-Ball .689 - .691
JT Maxim .689 - .691
JT Maxim 2 .688 - .690
JT Maxim 2 .689 - .691
JT Predator .689 - .691
Karnage Bite - .689
Karnage Tear - .691
Kick'n Paintballs .689 - .691
Kingman Java Classic .688 - .690
Kingman Java Supreme .689 - .691
Lazerball .690 - .691
Nelson Paintballs Affordaball .687 - .689
Nelson Anarchy .685
Nelson Challenger .685/.688
Nelson Dread: .688
Nelson Gold Medal .692
Nelson Hot Spot .691
Nelson Nel-Splat .691
Nelson Special Forces .685
Nitro Duck First Choice Field .688 - .690
Nitro Duck First Choice Slim fill .688 - .690
Nitro Duck First Choice Super Fill .688 - .690
Paintball Games Superball .692 and
Paintball Inc. Devil .691
Paintball Inc. Powerball .691
Paintball Inc. Proball .691
Paintball Inc. Proball Platinum .691
Paintball Junkies Ace .692
Paintball Junkies Joker .692
Paintball Junkies Morpheus .691
Paintball Junkies Mystic .690-.691
Paintball Junkies Reaper .692
Paintball Mania First Choice .692
Paintball Number 1 Premium Tourny .688 - .690
Pharmagel Tomahawk .690
PMI Advantage .688 - .690
PMI Big Ball .687 - .691
PMI Black Maxx .689 - .691
PMI Evil .689 - .691
PMI Invader .689 - .691
PMI Marballizer .685-.688
PMI Mercury .688 - .691
PMI Premium .689 - .691
Proball Devil 0.686 - 0.688 Small
Proball Lite 0.687 - 0.689 Medium
Proball 0.689 - 0.691 Medium
Proball Platinum 0.686 - 0.688 Small
RPS All Star .686 - .689
RPS Attitude .689 - .691
RPS Big Ball .689 - .691
RPS El Tigre .686 - .689
RPS EuroFlite .686 - .689
RPS Evil .686 - .689
RPS Flash .686 - .689
RPS Invader .689 - .691
RPS Lightening .686 - .689
RPS Marballizer .686 - .689
RPS Polar Ice .689 - .691
RPS Premium Gold .686 - .689
RPS Slamball .686 - .689
RPS SuperSwirl .686 - .689
Rufus Dawg T-Balls .688 - .690
Screamin' Banshe .688 - .690
Severe Paintballs Competitor .688 - .690
Severe Cyclone .688 - .690
Severe Hurricane .688 - .690
Severe Storm .688 - .690
Severe Tempest .688 - .690
Sheridan Premium .688 - .690
Sheridan Visible Impact .689 - .691
Smart Parts Death Paint .689 - .691
Smart Parts Smart Paint .689 - .691
Stinger Paintballs .688 - .690
TC Venom Paintballs .688 - .691
Tippmann Werks .689 - .691
Toroball Competition .688 - .690
Viewloader .689 - .691
Viewloader I-Blaz .689 - .691
Vortex Armageddon Series .687 - .689
WDP Victory .688 - .691
Winchester .688 - .691
WORR Competition .687
WORR Domination .687
WORR Perfection .687
WPN Toxic .688 - .689
XBall Bronze .688 - .689
XBall Silver .688 - .689
XBall Gold .688 - .689
Xtreme Aftermath .689 - .690
Xtreme Kryptonite (Glow in the dark) .689 - .690
Xtreme Legends .689 - .690
Xtreme Pointblank .689 - .691
Xtreme Steel .689 - .690
Zap Advantage .688 - .690
Zap Amp .689 - .691
Zap Chronic .689 - .691
Zap Chronic Ultimate .688 - .690
Zap Classic .689 - .691
Zap Performance Plus .688 - .690
Zap Primer .689 - .691
Zap Pro Series .688 - .690
Zap Pro European .689 - .691
Zap Rainbow .688 - .690
Zap Rec Series .688 - .690
Zap Seconds .688 - .690
Zap Select .689 - .691
Zap Spank .689 - .691
Zap Sport .689 - .691
Zap Tork .689 - .691
Zap Vapor .689 - .691
Zap X-Out .689 - .691
Zap ZXS Performance .688 - .690
Zap ZXS Rec Series .688 - .690
Zap ZXS Sport Series .688 - .690

Blow Test:

Ok, heres what you do, its called the "blow Test". take your barrel in your hand, get one of the paintballs you are using and put it into the barrel, tilt it forward, if it rolls down the barrel or out of the barrel, its too large of a bore, and that means the paint is to small. if you put it in and you blow your brains out its too big, if you blow it out easy, its still a little to small, if you can blow it out but it takes a few breaths, its too big. a good match occurs when you can blow briskly and the ball comes out with some force.

What makes a good barrel GOOD:

people still debate on this topic, but here are a few things you should know. Length IS important for they type of performance you want. if you want a quiet barrel you want lots of porting, now this uses some of the gas surrounding the ball, thus it doesnt travel as far, same for long barrels 16 inchs and up. they are long, causing friction, which slows the ball down. If you use Low Pressure you want little porting so the gas that is propeling the ball is not being wasted through the poritng. As far as accuracy, the smoother and the straighter the barrel is the more accurate the barrel will be. BUT you need a good paint match for this to hold true.

There are different types of barrels that will do different things.  The "Flatline" barrel for the Tippmann markers actually puts a physical backspin on the ball that enables it to fly further that most barrels.  The "Hammerhead" barrels are rifled (much like a regular gun barrel) that put a sideways sip on the ball to enable it to fly straighter and further than most.  Other companies claim to do the same, but remember that you get what you pay for.  If it's cheap, chances are it is for a reason.

If you are going to purchase a used barrel, you should ask if there's any damage.  If the barrel is bent or chipped on the inside this will probably render it useless.  If you don't care about aesthetics, then a few scratches are acceptable on the outside.

I know this isn't complete, but I hope it helps!