Today, there are two different paintball markers on the market, both classified by caliber. The most common and oldest gun uses the classic 0.68 caliber paintballs while newer versions have developed for the smaller .50cal type of ammunition. Manufacturers' choice to use smaller paintballs in their guns benefits the end user with smaller but also more realistic versions of these markers.
The most common gas used today is 12g co2 cartridges, which applies to most pistols. However, Tippmann's TiPX is one of these markers that can easily be converted to run on air with a simple modification.
Carrying a light side arm as a complement to your equipment is always a safe card and can save you from many tricky situations such as when you run out of paintballs in your marker and you need an ace in the rock sleeve to sort out the current situation.