According to this report the main cause of gun crime is, well, guess what? Guns.
Turns out the root causes of crime are the same sorts of things we always suspected, such as poverty, disaffected youth, all that jazz. But apparently, and this is the really shocking part, if you want to shoot someone you'll need a gun.
I know, I was shocked too. I've seen one of these 'gun' things, and I can't even see where you would put a copy of Grand Theft Auto in one.