A paintball marker, also known as a paintball gun, paint gun, or simply marker, is an air gun used in the shooting sport of paintball, and the main piece of paintball equipment. Paintball markers use compressed gas, such as carbon dioxide (CO2) or compressed air (HPA), to propel dye-filled gel capsules called paintballs through the barrel and quickly strike a target. The term "marker" is derived from its original use as a tool for forestry personnel to mark trees and ranchers to mark wandering cattle.