A barrel assembly for a pneumatic paintball gun includes a barrel body and a barrel insert. The barrel body has a breech end, a muzzle end, and a barrel bore extending longitudinally through the barrel body from the breech end to the muzzle end. The barrel insert has a breech end, a distal end, and an insert bore, and is configured to be removably housed within a breech portion of the bore of the barrel body. The breech end of the barrel body is configured to attach to a breech end of the paintball gun. The barrel insert collaborates with the barrel body to provide a firing bore for the barrel assembly which guides a paintball fired from the breech end of the gun through the barrel and out the muzzle end of the barrel body. The barrel insert can be constructed having a thin, flexible wall so as to permit lateral deformation of the barrel insert. This lateral flexibility enables the insert to dynamically adapt to the shape and diameter of the paintball, thereby increasing the efficiency of the energy transfer from the compressed gas to the paintball.