A barrel is the device on a marker, through which a controlled rapid expansion of gases are released in order to propel a paintball out of the end at a high velocity. Most after market barrels are rifled to give the paintball a spin.
Paintball barrel

A sniper barrel

The Length of Your BarrelEdit

Generally the longer the barrel, the more accurate and consistent your shots will be. But this advantage yields disadvantages. If you're barrel is longer, it take more propellant to get the ball through the barrel. Also, if you have a longer barrel, it can even slow down the ball slightly. A 16" barrel is recommended if you want to balance the advantages and disadvantages.

Cleaning Your BarrelEdit

Cleaning your gun is a quick process in which you can rinse it with water through it to rid of the clumps of paint/dirt in your barrel. Then you put the skinny end of your squeegee in your barrel and put it all the way through until your able to grab it on the other end. Then you should grab the other end and pull the squeegee all the way through it. But remember, your barrel will probably be as clean as your squeeqee is! So clean your squeegee every now and then.