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.