A hand held pneumatic gun (50) has an airway (60, 62) arranged to be coupled to a source of compressed air, the airway (60, 62) communicating with an air chamber (56) via a trigger valve assembly (64). The gun includes a detachable breech (96) that is supported in a sealed position against the air chamber (56) by a support ring (108) pivotally secured to the body (54) of the gun. An interlocking assembly (116) is arranged to interlock with the trigger valve assembly (64) if the breech (96) is not supported in its sealed position, so that actuation of the trigger valve assembly (64) is inhibited unless the breech (96) is sealed against the air chamber (56). The breech (96) incorporates a nozzle (98) that can be secured to an end of a hose or tube and a polymer projectile is placed into the breech (96) to be forced by the air pressure through the hose or tube to clean the interior of the hose or remove. Several improved types of projectile are also disclosed.