A paintball gun barrel structure, mainly for the proper adjustment of the inner diameter of the barrel of the paintball gun for firing paint balls according to the size of the paint balls when paint balls are loaded into the barrel, to keep proper point contact with the surfaces of the paint balls, and to prevent excess interference between the interior of the barrel and the paint balls, and to thusly enhance the stability of the ballistics of the interior of the barrel so that paint balls may be fired with high accuracy when being fired towards the barrel under the control of an operation unit.