The barrel attached to a paintball marker is crucial. It allows for the flow of the gas burst to launch the paintball; without it the ball would not travel far. Many factors influence the barrel's performance, listed here with descriptions.
The bottomline with barrels is that all aftermarket barrels will perform virtually the same. A barrel is simply a honed metal tube that you stick on the gun, unless one barrel is defective you're not going to see a huge difference with another.
Types of barrels:
There are several main types of barrels and barrel kits, I will list them here with a short description.
One-piece: Basic type of barrel, this is a straight-shot piece of material with minimal features.
Two-piece: This is a more dynamic barrel which screws apart to reveal two different pieces. The advantage to this is ability to swap out multiple backs and fronts for different reasons (length, color, bore size, etc). There are also efficiency benefits.
Multi-back barrel kit: This is a two-piece barrel that comes with an array of back sections instead of just one, each with a different bore size. You can then swap out the back for another of a different size, depending on the paint you shoot.
Insert barrel kit: These types of barrels accept a variable size "insert" which is placed in the back of the barrel. Like a multi-back kit, this comes with multiple bore sizes, which can be swapped at your convenience. The advantage to insert kits is that the inserts are cheaper and easier to produce than whole new backs, however they are possibly less reliable due to being thinner and easier to damage if you're not careful with them. The other advantage to insert kits is that you need only acquire another barrel in order to use your kit of inserts on another marker with different threads, whereas with a multi-back kit you'd need a new collection of backs for each thread.
Insert-based barrels can be one or two-piece. Example would be the Freak barrel (two-piece) or the Deadlywind Whisper barrel (one-piece) which also accepts Freak inserts.
From this point on I would like to organize information in terms of the barrel "from the ground up" as if it were being produced and examined in that regard.
Barrels have been made out of a large variety of materials over the years, including aluminum, ceramic, brass, carbon fiber, plastic/composite/polymer, stainless steel, titanium, kevlar, nickel, copper, glass (mostly for show purposes), plexiglass, and perhaps some others. The difference between materials is how heavy they are, how easy they are to produce, and how durable they are. Additional features are what type of color they can be made into, for instance being anodized or not. Most metals except for aluminum can only be made into one or two shades depending on what's done to them.
Probably the most common material [especially these days] is aluminum. It is hard enough to avoid being easily damaged, however still soft enough to be easily manufactured, and is also lightweight. It can be anodized into any variety of colors/patterns. Sometimes aluminum is used as the core to a polymer barrel, such as the J&J Ceramic Series.
Steel is another popular material for barrels. It is much more dense than aluminum so it is difficult to damage, however at the same time it is also a lot more costly to produce, and can only be made into one color (silver). It's also much heavier than anything else used out there. Many two-piece barrels come with a stainless back with a lightweight aluminum front, since the front can be made to match the marker's color scheme and be lightweight, not having to worry about being as durable.
Other metals include titanium which is lightweight yet stronger than steel, however is also more expensive and difficult to machine. Brass is the opposite; heavier than steel while being slightly less durable than aluminum. Neither titanium nor brass barrels are seen too much these days, since they both have their own distinct disadvantages (color options are limited as well). Brass barrels can tarnish, which is a good natural camouflage.
Materials such as plastic and carbon fiber have also sprung up over the past few years. Most plastic barrels are stock barrels for low-end markers, and rightly so. They scratch extremely easily and are very fragile. Carbon fiber, however, is stronger and easier to polish, although it is quite a bit more expensive to produce. Stiffis are an example of a 100% carbon fiber barrel.
The length of a barrel effects a number of things, and has no effect on some others. Notably, length will have a large bearing on how loud the shots sound when fired. Longer barrels are quieter, whereas shorter ones are louder.
Many independent tests have proven that a ball needs only a certain length to reach full acceleration, after that the rest is used to make the shot quieter and easier to aim. For most high pressure markers this length is around six inches. Most low pressure markers will need more length, however it can be taken up as a two-piece front with a larger bore. The exact length tends to be around 14-16 inches, though it varies depending on the marker and environment. If the ball doesn't have this amount of length in its barrel, it won't accelerate enough and you will have to increase your pressure/velocity to compensate. Alternately, if the barrel is longer than this length, you will also have to increase your pressure/velocity, due to added friction and compression being created. Of course most HP markers use longer barrels since 6" is much too loud for most people's tastes. The reason LP markers require more is because the air needs additional space to expand properly.
Another consideration of length is how easy the barrel will be to use for aiming and working around objects. Longer barrels tend to be easier to aim with (such as it is in this game). The other factor is that longer barrels will be easier to wrap around circular objects, and can be used to stick next to other objects if needed (such as through a pile of sticks). The disadvantage is, though, that longer barrels are a little more difficult to work with. They're easier to bang up against things, especially while you're moving around on the field. Some front-players swear against long barrels for this reason, others love them so they will be able to work up against the air bunker easily, it's all preference.
Accuracy is not based on length. See the bore size section for most details.
Bore size is used to control the general accuracy of the shot as well as a few smaller issues. An accurate match between bore size of the paintballs and the bore size of the barrel will most likely effect your accuracy more than anything else. Most barrels are considered either tight, standard, or loose bore, which translates into small, medium, or large, and nowadays come in bores ranging from 0.679-ca all the way 0.695. Most barrels are standard-sized which is usually between 0.687 and 0.690, however many series of barrels offer a selection of difference sizes available upon purchase. There are also many barrel kits made nowadays that offer a series of bore sizes, either through the use of switching out the barrel backs or by removing insertable sleeves through the backs (inserts).
The cause behind the accuracy of a proper bore to paint match is that, if there is extra space around the ball, the burst of air will not be as even as it should be. This will cause the ball to be pushed in one direction or another, which can influence a slight spin in that direction, leading the ball slightly off track once it exists the barrel. You may also loose some efficiency with a loose fit.
When the fit is too tight, you will lose some efficiency and will also increase the chance of the ball breaking in the barrel.
The trend nowadays is to swap out the back section of the barrel for another of different size, to match it up with paint correctly. This allows you to choose your desired bore size back depending on what you want to shoot.
Another similar concept is to use a smaller barrel sleeve or insert to control the effective bore of the barrel, and simply install and remove this piece into the actual external barrel as needed. Inserts are held in place using o-rings or sometimes just by very tight tolerances alone. The other advantage to an insert kit is that you need only change the barrel insert's housing in order to change to another barrel thread. Your one kit of inserts can be used on different markers so long as you have a back that fits them. Insert-based kits are also cheaper as well, since the inserts are easy to make. The disadvantage is that the inserts are possibly a bit less durable since they're made from thin metal.
The first insert-based barrel was the Smart Parts Freak, which is still largely around today, after being cosmetically redesigned a few times. Insert kit barrels can be two-piece or one-piece, depending on the options the manufacturer wants to offer. An example of a single-piece insert barrel is the Deadlywind Whisper, which is made to accept Freak inserts.
Internally, there is some work that needs to be done. Firstly, the barrel needs to be honed and polished so it will be smooth. Assuming the barrel bore is perfectly rigid (say, accurate to 0.0005" for instance) the smoothness of the internal surface will easily make or break the performance of the barrel.
Some manufacturers use other methods to obtain a good internal surface. barrels can be coated with teflon, ceramic, or a number of other materials and combinations therein. Example of this is a material Smart Parts uses on their barrels' bores. Due to this, SP barrel backs and inserts can't be reanodized without repolishing and/or recoating. It isn't advised to have a back anodized, generally speaking, even if they don't coat it with anything.
There is also the possibility of making the barrel bore unsmoothe. This would fall under the categories of either spiral rifling or linear rifling, both of which are not straight smooth internally. Linear porting involves multiple "tracks" which continue down the length of the barrel; the idea behind this is to reduce the contact surface area of the ball with the barrel to only the end of the tracks. There will be some efficiency loss though, and consistently produce linear tracks can be difficult. An example of this type of rifling would be the Core Rhino series barrel.
Spiral rifling is a method that involves twisted polygons that array themselves around the inside of the bore. The desired effect is to give the ball a slight spin however this is a somewhat circumstantial activity in terms of real performance benefits. An example of a spiral ported barrel is the Armson barrel series, which has 28 polygons around the internal surface, spiraled down the length.
External porting is also available on most barrels nowadays. Basically this involves drilling ports perpendicular or at an angle to the ball's flight. Porting is designed to ease the transition into the atmosphere by helping the expansion behind the ball as well as the compression in front of it, however there really isn't any hard evidence to support this either way. One thing it does to, however, is make the shots more quiet. Most barrels will have between two and eight lies of porting, in varying lengths and frequencies.
A related subject is a muzzle break which is a large amount of material removed from the front of the barrel, designed to reduce noise. Many companies manufacture or manufactured this at one point in time, although the actual noise loss isn't too much.