Marker Summary
A paintball marker, also known as a paintball gun, is the central or main piece of equipment in the sport of paintball. Markers make use of an expanding gas such as carbon dioxide, compressed air or nitrogen dioxide to propel paintballs through the barrel. The paintball community generally prefers to use the term "marker" rather than "gun" in order to mitigate the public perception that paintball markers are weapons, and that paintball is a dangerous sport. The term derives from its original use as a means for forestry personnel and ranchers to mark trees and wandering cattle.

The muzzle velocity of paintball markers can approach 300 feet per second (91 meters per second, or about 201 mph). Muzzle velocity above 300 feet per second is ruled unsafe in most commercial paintball fields. Below 300 fps, most paintballs will break upon impact without leaving significant damage beyond a small bruising.
Full Article