Supposing the US Government decided to ban all private ownership of Guns ( it's gonna happen eventually ) what then? Yes you can hide your gun but what about the cartridges? Make your own you are still going to need gun cotton etc even with gunpowder they'll probable trace the saltpeter. I have been thinking along the lines of a totally new type of weapon. They ain't never gonna ban gasoline. Why not use this as the propellent. I'm thinking along the lines of an air rifle with fuel injection and piezo-electric ignition. To get the same order of power you are going to need a large quantity of air. Imagine a pipe welded onto a mutch larger closed tube inside of which runs a piston. When you ignite by trigger the Air/Gas mixture the piston does not initially move ( heavy mass -grease gun machine pistol or temporary lock ) a ball bearing matching the pipe ID is shot out at target ( no rifling ) when its clear the piston starts to move against a spring sucking in a fresh charge of air via a one way valve. The piston runs back to a sear and is locked. When released by the trigger the piston moves forward under the spring compressing the air, at the final end of movement it initiates both a spring controlled fuel injector and piezo spark. The exploded mixture as well as driving ball and piston will have to reset the two springs and rotate a sleeve on the end of the pipe so that a new ball can be spring fed then return to seal off the pipe, the ballwill have to be held by spring loaded wedges. Fuel would be supplied from an attached gas tank. As well as windages problems there are issues of complexity and hitting force ( as well as penetration ) but it would be suitable as a short range machine gun.