An expansion chamber is an accessory for CO2 powered paintball markers. Its purpose is to increase accuracy and consistency of velocity, keeping your shots long in cold weather and knocking some of the sting out of warm weather shots.
It does this simply by giving the liquid CO2 room to expand to a gaseous state, which is beneficial because it keeps you from freezing your balls. It also regulates CO2 entering the bolt, which improves consistency and therefore improves accuracy. This is also a means of conserving CO2.
Expansion chamber prices range from about ten bucks for a simple gas-through grp to the full $49.95 for an all-out 22-stage chamber system. Normally expansion chambers replace the front grip or mount in place of the marker's original bottomline.