So because they are lethal, you want to outlaw them?
What about cars? Knives? Rocks? If someone is dumb enough to blow themselves up with a shabby spud gun, they deserve to go down Ala Darwin. If they cause destruction with it, then they deserve to get punished. And a gun with a bore over half an inch (Exceptions are black powder firearms, and they have limits also) that shoots a projectile that is anti-personnel is a destructive device, and regulated by ATF.
Tennis balls moving at Mach 3 hurt like hell, but usually don't kill.