Depending on the state, getting a Destructive Device tax stamp and local law enforcement approval, Americans can currently privately own
fully-functional, diesel-powered traverse, towed, anti-aircraft 40 mm auto-cannons if they were made before some treaty (I can’t remember which one). They’re several hundred thousand dollars and the shells are hundreds of dollars each. That’s right: moderately rich people can own their very own artillery in some states like Arizona.
http://youtu.be/BcJEFosXfFQ