The Richardson Industries M5, more commonly known as the Richardson Philippine Guerilla Gun, is a very simple shotgun.
Iliff D. Richardson opened his own manufacturing corporation, Richardson Industries in 1946 to manufacture this gun. It was very cheap and cost $7 in 1942 (about $80 in today's money with inflation).
The M5 was basically just a hunk of wood with a barrel on it. The gun lacked a trigger, hammer, extractor, safety, magazine, choke tubes, iron sights, or rifling. The wood used to make the stock felt like a mid-grade 2x4 piece of wood. The barrel was basically a smoothbore steel tube, while the trigger was nothing much more than a slightly larger guide tube with a fixed "firing pin" in the rear. To load the gun, the barrel was pulled out, and a 12 gauge shell was loaded into it. The barrel would then be put back in the receiver. Firing the weapon requires the user to slam the barrel rearward fairly hard; the fixed firing pin hits the primer and fires the gun.
There were 2 variants of the gun: a basic version and a deluxe version.
This was the standard version of the M5. It lacked a trigger or handguard and cost about $7. As the gun lacked a conventional safety, the gun used a wingnut to restrict barrel movement. This model had only five parts.
This was a better built model of the M5. The Deluxe version had a trigger, trigger guard and foregrip, but the trigger was not exactly used to fire the weapon; it was actually a lever. Pulling the "trigger" releases a locking wedge in the barrel, allowing barrel movement. This model had seventeen parts.