a compressed gas firearm is capable of firing either cartridges filed with pellets , or solid unitary projectiles . the firearm includes a barrel that is hinged to the butt so that cartridges or an adapter can be inserted into the end of the barrel . a compressed gas chamber in the butt is capable of releasing compressed gas against the rear of the cartridge or adapter to fire the pellets or the unitary projectile . when the adapter is inserted into the barrel , a projectile can then be inserted into the adapter before the barrel is closed for firing . indentations may be formed in the rear of the barrel to facilitate removal of the cartridges or adapter .