A quick and dirty improvised firearm assembled using pipe fittings available at any hardware store. Ammunition in this case has been improvised using nail gun charges with the crimp removed and a lead projectile inserted. The weapon functions by pulling back the ball end of the spring loaded striker bar and releasing to fire.
An example of a criminally used improvised 12 gauge pipe shotgun. The front tube houses the barrel while the rear tube contains a spring and a striker which is pulled to the rear and released to fire. A bolt attached to the front of the rear piece allows the gun to be locked and unlocked in a bayonet type fashion for loading and unloading.
Two zip guns seized by police which are assembled from readily available pipe fittings. Both consist of a threaded copper pipe with a brass hose nipple for a barrel. A striker attached to a spring and a length of wire rope is pulled back and released to fire.
Similar to the classic shotgun variation though arranged in reverse. It is made from three components which include a barrel pipe welded into a larger pipe and a length of steel bar with a nail in the end to act as a forward driven firing pin. Simple and highly effective.