The first thing most players change on their paintgun is the barrel. Factory barrels that come with most paintguns work O.K., but they are not in the same league with high-end barrels from companies like Check It, LAPCO, B.O.A., Smart Parts etc. Barrels from companies like these usually have some special design features like rifling, muzzle brakes, variable bore diameter etc., and they are made to very high standards. For example, the smoother the finish on the inside of a barrel, the better. A smoother finish means less friction, and that produces better range and accuracy. The sort of honing and polishing required to get the glass-smooth finish found in the best barrels takes a lot of time and effort, which is one reason they cost more. If cost is a factor for you right now, stick with the factory barrel until you can afford something really good that will give you a real increase in performance, rather than buying a so-so barrel that won't make much of a difference anyway..

Ask other players with the same kind of paintgun as you why they chose the barrel they did. Maybe they'll even let you try it out.
You'll find that some barrels are quieter than others, or maybe seem to be more accurate with a particular brand of paint.
Take into account the kind of paint you normally use. There is no point buying a barrel that likes Brand X paintballs if all the fields in your area use Brand Y. And keep in mind that even a great barrel will yield disappointing results if you use poor quality paint.

Another very popular accessory is a "Venturi" bolt. A venturi design uses several small gas ports in the face of the bolt rather than one large port.
The idea is to spread the blast of gas more evenly over the surface of the paintball. This not only makes it less likely that the shell of the paintball will "Blow Out" (leaving a mess in your barrel) but also is said to distort the elastic shell of the ball less, resulting in improved range and accuracy.
There are several brands of venturi bolts available for the more popular paintguns, and for the most part they all work pretty well. But be careful when buying a venturi bolt for the Pro-lite.
We carry only the Check It Rimfire bolt for this paintgun because it is the only reliable design we have found for the Pro-lite.

Expansion chambers are also popular for semi-auto paintguns. An expansion chamber prevents liquid CO2 from the tank from reaching the paintgun.
This improves accuracy by providing more consistent velocities, and increases efficiency as well, giving you more shots per tank. Some semis like the AutoMag and Autococker will malfunction if liquid CO2 gets into their operating mechanism, so an expansion chamber is a worthwhile addition to these paintguns.
Generally, the larger an expansion chamber is, the better, so avoid those tiny "micro" expansion units, as they just don't do much. Most expansion chambers require additional parts like fittings, elbows, hoses and so on to install, so be certain you are going to like all that extra hardware bolted onto your paintgun.
Many paintguns now have vertical tank adapters which make it easy to install expansion setups, with the tank used in either a bottomline configuration or with a remote setup. I&I carries complete expansion setups with all the hardware needed for all the more popular paintguns like the Spyder, AutoMag, VM-68, Autococker etc.

Speaking of bottomlines, if you are using a horizontal tank on the gun, you should consider having "Anti-siphon" fittings installed in your tanks at a cost of about $15.00 per tank.
An anti-siphon fitting keeps the liquid CO2 in the tank where it belongs, and allows only gaseous CO2 to reach the paintgun. However, anti-siphon tanks must be matched to the paintgun they are used with, unless you have one of our new Universal Anti-Siphon Adapters, which allows any anti-siphon tank to be used on any paintgun.
Anti-siphon tanks should not be used in a vertical postion like in a vertical tank adapter on in a vertical remote tank harness.